Light-emitting device and electronic device using the same

ABSTRACT

It is an object to provide a light-emitting device and an electronic device which can provide an image with excellent image quality. One of the present inventions is a light-emitting device including a plurality of light-emitting elements each exhibiting a different emission color. At least one of the plurality of light-emitting elements has n light-emitting layers (n is a natural number, n≧2) between a pair of electrodes. Further, at least one of the n light-emitting layers includes a substance which provides emission from a triplet excitation state. In a light-emitting device having such a structure, an image is displayed by combining emissions from the plurality of light-emitting elements.

This application is a continuation of copending U.S. application Ser.No. 11/301,706, filed on Dec. 13, 2005 which is incorporated herein byreference.

TECHNICAL FIELD

The present invention relates to a light-emitting device using alight-emitting element as a pixel, and an electronic device using such alight-emitting device as a display device.

BACKGROUND OF THE INVENTION

In recent years, a light-emitting device using, as a pixel, alight-emitting element which emits light by current excitation has beendeveloped actively. Such light-emitting devices are incorporated inelectronic devices such as a television image receiver, and are known asdevices which provide images to users of the electronic devices.

In a light-emitting device, light-emitting elements for red, blue andgreen are provided. Light emitted from each of the light-emittingelements is combined while changing luminance, emission time and thelike, thereby an image having different brightness, chromaticity, orgradation is displayed.

Most of light-emitting devices which have ever been developed havestructures of a combination of red light, blue light and green light.However, for example, as described in Patent Document 1, a displaydevice having a structure provided with a light-emitting element foremitting white emission has been developed.

-   Patent Document 1—Japanese Patent Laid-Open No. 2000-200061 bulletin

It is an object of the present invention to provide a light-emittingdevice and an electronic device which can provide an image withexcellent image quality.

SUMMARY OF THE INVENTION

One embodiment of the present invention is a light-emitting deviceincluding a plurality of light-emitting elements each exhibiting adifferent emission color. At least one of the plurality oflight-emitting elements has n light-emitting layers (n is a naturalnumber, n≧2) between a pair of electrodes. In a light-emitting devicehaving such a structure, an image is displayed by combining emissionsfrom the plurality of light-emitting elements.

One embodiment of the present invention is a light-emitting deviceincluding a plurality of light-emitting elements each exhibiting adifferent emission color. At least one of the plurality oflight-emitting elements has n light-emitting layers (n is a naturalnumber, n≧2) between a pair of electrodes. Further, at least one of then light-emitting layer includes a substance which provides emission froma triplet excitation state. In a light-emitting device having such astructure, an image is displayed by combining emissions from theplurality of light-emitting elements.

One embodiment of the present invention is a light-emitting deviceincluding a plurality of light-emitting elements each exhibiting adifferent emission color. At least one of the light-emitting elementshas n light-emitting layers (n≧2, a natural number) between a pair ofelectrodes. In the n light-emitting layers, between a m-thlight-emitting layer (m is a natural number) and a (m+1)-thlight-emitting layer, a layer is provided, which has a larger ionizationpotential than an ionization potential of the m-th light-emitting layerand which has a smaller electron affinity than an electron affinity ofthe (m+1)-th light-emitting layer. In the light-emitting device havingsuch a structure of the present invention, an image is displayed bycombining emissions from the light-emitting elements.

One embodiment of the present invention is a light-emitting deviceincluding a plurality of light-emitting elements each exhibiting adifferent emission color. At least one of the light-emitting elementshas n light-emitting layers (n≧2, n is a natural number) between a pairof electrodes. Further, at least one of the n light-emitting layersincludes a substance exhibiting emission from a triplet excited state.In the n light-emitting layers, between a m-th light-emitting layer (mis a natural number) and a (m+1)-th light-emitting layer, a layer isprovided, which has a larger ionization potential than an ionizationpotential of the m-th light-emitting layer and which has a smallerelectron affinity than an electron affinity of the (m+1)-thlight-emitting layer. In the light-emitting device having such astructure of the present invention, an image is displayed by combiningemissions from the light-emitting elements.

One embodiment of the present invention is a light-emitting deviceincluding a plurality of light-emitting elements each exhibiting adifferent emission color. At least one of the light-emitting elementshas n light-emitting layers (n≧2, a natural number) between a pair ofelectrodes. In the n light-emitting layers, between a m-thlight-emitting layer (m is a natural number) and a (m+1)-thlight-emitting layer, a layer is provided, which includes an aromaticamine compound as well as a metal oxide including a metal atom belongingto any of Groups 4 to 8. In the light-emitting device having such astructure of the present invention, an image is displayed by combiningemissions from the light-emitting elements.

One embodiment of the present invention is a light-emitting deviceincluding a plurality of light-emitting elements each exhibiting adifferent emission color. At least one of the light-emitting elementshas n light-emitting layers (n≧2, n is a natural number) between a pairof electrodes. Further, at least one of the n light-emitting layersincludes a substance exhibiting emission from a triplet excited state.Between a m-th light-emitting layer (m is a natural number) and a(m+1)-th light-emitting layer, a layer is provided, which includes anaromatic amine compound as well as a metal oxide including a metal atombelonging to any of Groups 4 to 8. In the light-emitting device havingsuch a structure of the present invention, an image is displayed bycombining emissions from the light-emitting elements.

In the light-emitting device described above, respective light-emittingsubstances included in the n light-emitting layers may exhibit differentemission colors.

One embodiment of the present invention is that, in the light-emittingdevice described above, a plurality of power source potentials areincluded and to the light-emitting elements each exhibiting a differentemission color, respective different power source potentials areapplied. Further, the plurality of power source potentials are generatedby a voltage step-up circuit.

In the light-emitting device described above, a charge pump may be usedas the voltage set-up circuit.

One embodiment of the present invention is that in the light-emittingdevice described above, the charge pump including a constant voltagesource; a first rectifying element to an N-th rectifying element (N≧2, Nis a natural number); and a first capacitor element to an N-th capacitorelement. And the first rectifying element to the N-th rectifying elementare connected in series, and an input terminal of the first rectifyingelement is connected to the constant voltage source. One electrode ofeach of the first capacitor element to the N-th capacitor element isconnected to an output terminal of each of the first rectifying elementto the N-th rectifying element. A clock pulse or a clock inverted pulseis inputted alternately to the other electrode of each of the firstcapacitor element to the (N−1)-th capacitor element, and a predeterminedpotential is inputted to the other electrode of the N-th capacitorelement. A power source potential to be applied to the light-emittingelement is obtained from an arbitrary one electrode of each of the firstcapacitor element to the N-th capacitor element.

One embodiment of the present invention is a light-emitting deviceincluding a first light-emitting element including a firstlight-emitting layer between a pair of electrodes; a secondlight-emitting element including a second light-emitting layer between apair of electrodes; a third light-emitting element including a thirdlight-emitting layer between a pair of electrodes; and a fourthlight-emitting element including the first light-emitting layer, thesecond light-emitting layer and the third light-emitting layer between apair of electrodes. Here, the fourth light-emitting element is adjacentto each of the first light-emitting element, the second light-emittingelement and the third light-emitting element. A plurality of groups eachhaving the first light-emitting element, the second light-emittingelement, the third light-emitting element and the fourth light-emittingelement as one set are arranged, and each of the groups serves as apixel.

One embodiment of the present invention is a television image receiverin which a module including a light-emitting device and a circuit boardis incorporated in a casing. Here, the light-emitting device includes aplurality of light-emitting elements each exhibiting a differentemission color. And at least one of the light-emitting elements has nlight-emitting layers (n≧2, n is a natural number) between a pair ofelectrodes. In the television image receiver including thelight-emitting device with such a structure, an image is displayed bycombining emissions from the light-emitting elements.

One embodiment of the present invention is a television image receiverin which a module including a light-emitting device and a circuit boardis incorporated in a casing. Here, the light-emitting device includes aplurality of light-emitting elements each exhibiting a differentemission color. And at least one of the light-emitting elements has nlight-emitting layers (n≧2, n is a natural number) between a pair ofelectrodes. At least one of the light-emitting layers includes asubstance exhibiting emission from a triplet excited state. In thetelevision image receiver including the light-emitting device with sucha structure, an image is displayed by combining emissions from thelight-emitting elements.

One embodiment of the present invention is a television image receiverin which a module including a light-emitting device and a circuit boardis incorporated in a casing. Here, the light-emitting device includes aplurality of light-emitting elements each exhibiting a differentemission color. And at least one of the light-emitting elements has nlight-emitting layers (n≧2, n is a natural number) between a pair ofelectrodes. In the n light-emitting layers, between a m-thlight-emitting layer (m is a natural number) and a (m+1)-thlight-emitting layer, a layer is provided, which has a larger ionizationpotential than an ionization potential of the m-th light-emitting layerand which has a smaller electron affinity than an electron affinity ofthe (m+1)-th light-emitting layer. In the television image receiverincluding the light-emitting device with such a structure, an image isdisplayed by combining emissions from the light-emitting elements.

One embodiment of the present invention is a television image receiverin which a module including a light-emitting device and a circuit boardis incorporated in a casing. Here, the light-emitting device includes aplurality of light-emitting elements each exhibiting a differentemission color. And at least one of the light-emitting elements has nlight-emitting layers (n≧2, n is a natural number) between a pair ofelectrodes. At least one of the light-emitting layers includes asubstance exhibiting emission from a triplet excited state. In the nlight-emitting layers, between a m-th light-emitting layer (m is anatural number) and a (m+1)-th light-emitting layer, a layer isprovided, which has a larger ionization potential than an ionizationpotential of the m-th light-emitting layer and which has a smallerelectron affinity than an electron affinity of the (m+1)-thlight-emitting layer. In the television image receiver including thelight-emitting device with such a structure, an image is displayed bycombining emissions from the light-emitting elements.

One embodiment of the present invention is a television image receiverin which a module including a light-emitting device and a circuit boardis incorporated in a casing. Here, the light-emitting device includes aplurality of light-emitting elements each exhibiting a differentemission color. And at least one of the light-emitting elements has nlight-emitting layers (n≧2, n is a natural number) between a pair ofelectrodes. In the n light-emitting layers, between a m-thlight-emitting layer (m is a natural number) and a (m+1)-thlight-emitting layer, a layer is provided, which includes an aromaticamine compound as well as a metal oxide including a metal atom belongingto any of Groups 4 to 8. In the television image receiver including thelight-emitting device with such a structure, an image is displayed bycombining emissions from the light-emitting elements.

One embodiment of the present invention is a television image receiverin which a module including a light-emitting device and a circuit boardis incorporated in a casing. Here, the light-emitting device includes aplurality of light-emitting elements each exhibiting a differentemission color. And at least one of the light-emitting elements has nlight-emitting layers (n≧2, n is a natural number) between a pair ofelectrodes. At least one of the light-emitting layers includes asubstance exhibiting emission from a triplet excited state. In the nlight-emitting layers, between a m-th light-emitting layer (m is anatural number) and a (m+1)-th light-emitting layer, a layer isprovided, which includes an aromatic amine compound as well as a metaloxide including a metal atom belonging to any of Group 4 to 8. In thetelevision image receiver including the light-emitting device with sucha structure, an image is displayed by combining emissions from thelight-emitting elements.

In the television image receiver described above, respectivelight-emitting substances included in the n light-emitting layers mayexhibit different emission colors.

One embodiment of the present invention is a television image receiverin which a module including a light-emitting device and a circuit boardis incorporated in a casing. The light-emitting device includes alight-emitting element including a first light-emitting layer, a secondlight-emitting layer and a third light-emitting layer between a pair ofelectrodes. In such a light-emitting element, the first light-emittinglayer includes a light-emitting substance exhibiting reddish emission,the second light-emitting layer includes a light-emitting substanceexhibiting greenish emission, and the third light-emitting layerincludes a light-emitting substance exhibiting bluish emission.

One embodiment of the present invention is a television image receiverin which a module including a light-emitting device and a circuit boardare incorporated in a casing. The light-emitting device includes alight-emitting element including a first light-emitting layer and asecond light-emitting layer between a pair of electrodes. In such alight-emitting element, the first light-emitting layer includes alight-emitting substance exhibiting reddish yellow emission and thesecond light-emitting layer includes a light-emitting substanceexhibiting bluish emission.

One embodiment of the present invention is a television image receiverin which a module including a light-emitting device and a circuit boardis incorporated in a casing. Here, the light-emitting device includes alight-emitting element including a first light-emitting layer and asecond light-emitting layer between a pair of electrodes. In such alight-emitting element, the first light-emitting layer includes alight-emitting substance exhibiting reddish emission and the secondlight-emitting layer includes a light-emitting substance exhibitingblue-greenish emission.

In the present invention, a reddish color means a color whose coordinateis in the range where x is 0.60 or more and y is 0.40 or less in thechromaticity diagram when adopting CIE-XYZ color system. A greenishcolor means a color whose coordinate is in the range where x is 0.30 orless and y is 0.60 or more. in the chromaticity diagram when adoptingCIE-XYZ color system. A bluish color means a color whose coordinate isin the range where x is 0.20 or less and y is 0.20 or less in thechromaticity diagram when adopting CIE-XYZ color system. In addition, ablue-green color means a color whose coordinate is in the range where xis 0.20 or less and y is 0.25 or more but 0.5 or less in thechromaticity diagram when adopting CIE-XYZ color system. Note that theCIE-XYZ color system is a color system based on tristimulus values X, Y,and Z. Further, a reddish yellow based color means a color whosecoordinate is in the range where x is 0.40 or more but 0.55 or less andy is 0.40 or more in the chromaticity diagram when adopting CIE-XYZcolor system. Note that the CIE-XYZ color system is a color system basedon tristimulus values X, Y, and Z. The chromaticity diagram shows colorsby a coordinate space of x and y based on tristimulus values X, Y, andZ. Chromaticity is the color type from which brightness is excepted,which is quantitatively defined.

By implementing the present invention, a light-emitting device can beprovided which has a light-emitting element having a higher luminancewhen an arbitrary current flows thereto. In addition, by incorporatingsuch a light-emitting device, a television image receiver with excellentimage quality can be obtained.

By implementing the present invention, the luminance of light extractedto the outside when an arbitrary current flows thereto can be adjustednot only by a value of a current supplied to a light-emitting element,but also by changing the number of light-emitting layers included in onelight-emitting element. Thus, a light-emitting device which can displaya favorable image with good balance of luminance of each color, can beobtained. By incorporating such a light-emitting device, a televisionimage receiver with excellent image quality can be obtained.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1(A)-1(C) are diagrams showing modes of a light-emitting elementincluded in a light-emitting device of the present invention.

FIGS. 2(A)-2 (C) are diagrams showing modes of a light-emitting elementincluded in a light-emitting device of the present invention.

FIG. 3 is a diagram showing a mode of a light-emitting element includedin a light-emitting device of the present invention.

FIG. 4 is a diagram showing a mode of a light-emitting element includedin a light-emitting device of the present invention.

FIGS. 5(A)-5(B) are view showing a mode of light-emitting elementsincluded in a light-emitting device of the present invention.

FIGS. 6(A)-6(B) are views showing modes of a light-emitting elementincluded in a light-emitting device of the present invention.

FIGS. 7(A)-7(D) are views showing a mode of light-emitting elementsincluded in a light-emitting device of the present invention.

FIGS. 8(A)-8(B) are views showing a mode of a light-emitting device ofthe present invention.

FIG. 9 is a view showing a mode of a light-emitting device of thepresent invention.

FIG. 10 is a view showing a mode of an electronic device to which thepresent invention is applied.

FIG. 11 is a diagram showing a mode of an electronic device to which thepresent invention is applied.

FIG. 12 is a view showing a mode of an electronic device to which thepresent invention is applied.

FIGS. 13(A)-13(B) are views showing modes of a light-emitting device ofthe present invention.

FIG. 14 is a diagram showing a mode of a light-emitting element includedin a light-emitting device of the present invention.

FIG. 15 is a diagram showing an example of a pixel configuration.

FIG. 16 is a schematic diagram of a signal line driver circuit describedin Embodiment 1.

FIG. 17 is a schematic diagram of a light-emitting device described inEmbodiment 1.

FIG. 18 is a diagram showing a pixel configuration of a light-emittingdevice described in Embodiment 1.

FIG. 19 is a diagram showing a voltage step-up circuit.

FIG. 20 is a diagram showing a voltage step-up circuit.

FIG. 21 is a diagram showing a voltage step-up circuit.

FIGS. 22(A)-22(B) are diagrams showing stabilized power source circuits.

FIG. 23 is a diagram showing a digital-analog conversion circuit.

FIG. 24 is a schematic diagram of pixels in the case where a powersource potential of a power source line of a pixel for each colorelement is common.

FIG. 25 is a schematic diagram of a voltage step-up circuit.

FIG. 26 is a circuit diagram having a function of a clock pulse.

FIG. 27 is a diagram showing a voltage step-down circuit.

DETAILED DESCRIPTION OF THE PRESENTLY PREFERRED EMBODIMENTS

Hereinafter, one mode of the present invention will be described.However, the present invention can be carried out in many differentmodes, and it is easily understood by those skilled in the art thatmodes and details herein disclosed can be modified in various wayswithout departing from the spirit and the scope of the presentinvention. Therefore, it should be noted that the present inventionshould not be interpreted as being limited to the description of theembodiment modes.

Embodiment Mode 1

A mode of a light-emitting element included in a light-emitting deviceof the present invention will be described with reference to FIGS. 1(A)to 1(C). In the light-emitting element of the present invention, a firstlight-emitting element shown in FIG. 1(A), a second light-emittingelement shown in FIG. 1(B), a third light-emitting element shown in FIG.1(C) are provided. Each light emitted from the first light-emittingelement, the second light-emitting element, and the third light-emittingelement is different in color. There are no particular limitations onthe color of light emitted from the light-emitting elements. In thisembodiment mode, description is made on a case where the firstlight-emitting element emits reddish light, the second light-emittingelement emits greenish light, and the third light-emitting element emitsbluish light.

In the light-emitting element shown in FIG. 1(A), a first light-emittinglayer 111 a and a second light-emitting layer 111 b are provided betweenthe first electrode 101 and the second electrode 102. Between the firstlight-emitting layer 111 a and the second light-emitting layer 111 b, afirst layer 121 is provided. The first layer 121 serves as a layer forpreventing excitation energy from moving only in any one direction ofthe first light-emitting layer 111 a and the second light-emitting layer111 b, or an electron generation layer.

First, described is a case where the first layer 121 serves as a layerfor preventing excitation energy from moving only in any one directionof the first light-emitting layer 111 a and the second light-emittinglayer 111 b. In this case, the light-emitting element operates asfollows: When a voltage is applied to the first electrode 101 and thesecond electrode 102 such that a potential of the first electrode 101becomes higher than that of the second electrode 102, holes are injectedto the first layer 121 from the first electrode 101 side, and electronsare injected to the first layer 121 from the second electrode 102 side.In at lease one layer of the first layer 121, the first light-emittinglayer 111 a, and the second light-emitting layer 111 b, electrons andholes are recombined to generate excitation energy. Light-emittingsubstances included in the first light-emitting layer 111 a and thesecond light-emitting layer 111 b are excited by the generatedexcitation energy thus generated, and then, light is emitted when itreturns to a ground state. In the light-emitting layer 121, the firstlayer 121 has a larger ionization potential than an ionization potentialof the first light-emitting layer 111 a, and is preferably a layerhaving a higher LUMO level than a LUMO level in the secondlight-emitting layer 111 b. In addition, the first layer 121 ispreferably formed so as to have a thickness of 1 nm to 30 nm. Byproviding the first layer 121 between the first light-emitting layer 111a and the second light-emitting layer 111 b, it can be prevented thatthe excitation energy moves only in any one direction of the firstlight-emitting layer 111 a and the second light-emitting layer 111 b.

Here, light-emitting substance included in each of the firstlight-emitting layer 111 a and the second light-emitting layer 111 b isnot limited especially. It may be a substance which emits fluorescence,or a substance which emits phosphorescence. Here, each of the firstlight-emitting layer 111 a and the second light-emitting layer 111 b maybe a layer including only a light-emitting substance or a layer in whicha light-emitting substance is dispersed in a substance having a largerenergy gap than the light-emitting substance. In the present invention,a light-emitting substance refers to a substance which has a favorableemission efficiency and which can emit light in a desired emissionwavelength. As specific examples of a light-emitting element which canbe used in a case of emitting reddish light, like the firstlight-emitting element, the following substances exhibiting emissionspectrum with peaks at 600 to 680 nm can be employed:

4-dicyanomethylene-2-isopropyl-6-[2-(1,1,7,7-tetramethyljulolidine-9-yl)ethenyl]-4H-pyran(abbreviated to DCJTI);4-dicyanomethylene-2-methyl-6-[2-(1,1,7,7-tetramethyljulolidine-9-yl)ethenyl]-4H-pyran(abbreviated to DCJT);4-dicyanomethylene-2-tert-butyl-6-[2-(1,1,7,7-tetramethyljulolidine-9-yl)ethenyl]-4H-pyran(abbreviated to DCJTB); periflanthene;2,5-dicyano-1,4-bis[2-(10-methoxy-1,1,7,7-tetramethyljulolidine-9-yl)ethenyl]benzeneand the like, are given. In addition to the substances which emitsfluorescence, a material which emits phosphorescence, such as(acetylacetonato)bis[2-(2-benzothienyl)pyridinato]iridium(III)(abbreviated to Ir(btp)₂(acac)), or(2,3,7,8,12,13,17,18-octaethyl-21H,23H-porphyrinato)platinum(II) can beused as the light-emitting substance which provides reddish light. Notethat the light-emitting substances included in the first light-emittinglayer 111 a and the second light-emitting layer 111 b may be the same ordifferent. A substance which is included in the first light-emittinglayer 111 a or the second light-emitting layer 111 b together with thelight-emitting substance and which is used for dispersing thelight-emitting substance is not limited especially, and it may beselected appropriately in consideration of an energy gap or the like ofa substance to be used as the light-emitting substance. For example, ananthracene derivative such as 9,10-di(2-naphthyl)-2-tert-butylanthracene(abbreviated to t-BuDNA); a carbazole derivative such as4,4′-bis(N-carbazolyl)biphenyl (abbreviated to CBP); a quinoxalinederivative such as 2,3-bis(4-diphenylaminophenyl)quinoxaline(abbreviated to TPAQn),2,3-bis{4-[N-(1-naphthyl)-N-phenylamino]phenyl}-dibenzo[f,h]quinoxaline(abbreviated to NPADiBzQn); a metal complex such asbis[2-(2-hydroxyphenyl)pyridinato]zinc (abbreviated to Znpp₂) andbis[2-(2-hydroxyphenyl)benzoxazolato]zinc (abbreviated to ZnBOX₂), orthe like can be used with the light-emitting substance.

In addition, a hole transporting layer 122 may be provided between thefirst light-emitting layer 111 a and the first electrode 101 as shown inFIG. 1. Here, the hole transporting layer is a layer having a functionof transporting holes injected from the first electrode 101 side to thefirst light-emitting layer 111 a side. Like this, by providing the holetransporting layer 122, the distance between the first electrode 101 andthe first light-emitting layer 111 a can be increased. As a result, itcan be prevented that light is quenched due to a metal included in thefirst electrode 101. The hole transporting layer is preferably formedusing a material having a high hole transporting property, inparticular, preferably formed using a substance having a hole mobilityof 1×10⁻⁶ cm²/Vs or more. Note that the substance having a high holetransporting property is a substance which has a higher hole mobilitythan an electron mobility, and in which the ratio value of hole mobilityto electron mobility (=hole mobility/electron mobility) is 100 or more.As specific examples of a substance which can be used for forming thehole transporting layer 122,4,4′-bis[N-(1-naphthyl)-N-phenylamino]biphenyl (abbreviated to NPB);4,4′-bis[N-(3-methylphenyl]-N-phenylamino]biphenyl (abbreviated to TPD);4,4′,4″-tris(N,N-diphenylamino)triphenylamine (abbreviated to TDATA);4,4′,4″-tris[N-(3-methylphenyl)-N-phenylamino]triphenylamine(abbreviated to MTDATA); and4,4′-bis{N-[4-(N,N-di-m-tolylamino)phenyl]-N-phenylamino}biphenyl(DNTPD); 1,3,5-tris[N,N-di(m-tolyl)amino]benzene (abbreviated tom-MTDAB); 4,4′,4″-tris(N-carbazolyl)-triphenylamine (abbreviated toTCTA); phthalocyanine (abbreviated to H₂Pc); copper phthalocyanine(abbreviated to CuPc); (vanadyl phthalocyanine (abbreviated to VOPc),and the like are given. In addition, the hole transporting layer 122 mayhave a multilayer structure including two or more layers made of thesubstances described above.

Furthermore, a hole-injecting layer may be provided between the firstelectrode 101 and the hole-transporting layer 122 as shown in FIG. 1.The hole-injecting layer is a layer having a function to assist holes tobe injected to the hole-transporting layer 122 from the first electrode101. By providing the hole-injecting layer, the ionization potentialdifference between the first electrode 101 and the hole-transportinglayer 122 is relieved; thus, holes are easily injected. Thehole-injecting layer is preferably formed using a substance of whichionization potential is lower than that of a substance forming thehole-transporting layer 122 and higher than that of a substance formingthe first electrode 101 or using a substance of which energy band curveswhen provided as a thin film of 1 nm to 2 nm between thehole-transporting layer 122 and the first electrode 101. As for aspecific example of a substance that can be used to form thehole-injecting layer, a phthalocyanine-based compound such asphthalocyanine (abbreviated to H₂Pc) or copper phthalocyanine (CuPc), apolymer such as poly (ethylenedioxythiophene)/poly (styrenesulfonicacid) solution (PEDOT/PSS), and the like can be given. That is, byselecting a substance so that an ionization potential of the holeinjecting layer becomes comparatively smaller than an ionizationpotential of the hole transporting layer 122, the hole injecting layercan be formed. Note that in the case of providing the hole injectinglayer, the first electrode 101 is preferably formed using a substancewith a high work function such as indium tin oxide.

In addition, an electron transporting layer 123 may be provided betweena second electrode 102 and a second light-emitting layer 111 b as shownin FIG. 1. Here, the electron transporting layer has a function oftransporting electrons injected from the second electrode 102 to thesecond light-emitting layer 111 b side. By providing the electrontransporting layer 123 in this manner, the distance between the secondelectrode 102 and the second light-emitting layer 111 b can beincreased. As a result, it can be prevented that light is quenched dueto a metal included in the second electrode 102. The electrontransporting layer is preferably faulted using a substance having a highelectron transporting property, in particular, preferably formed using asubstance having an electron mobility of 1×10⁻⁶ cm²/Vs or more ispreferable. Note that the substance having high hole transportingproperty indicates a substance having higher mobility of electrons thanthat of holes, where a ratio value of electron mobility to hole mobility(=electron mobility/hole mobility) is more than 100. As a specificexample of a substance that can be used to form theelectron-transporting layer 123:2-(4-biphenylyl)-5-(4-tert-buthylphenyl)-1,3,4-oxadiazole (abbreviatedto PBD); 1,3-bis[5-(p-tert-buthylphenyl)-1,3,4-oxadiazole-2-yl]benzene(abbreviated to OXD-7);3-(4-tert-buthylphenyl)-4-phenyl-5-(4-biphenylyl)-1,2,4-triazole(abbreviated to TAZ);3-(4-tert-buthylphenyl)-4-(4-ethylphenyl)-5-(4-biphenylyl)-1,2,4-triazole(abbreviated to p-EtTAZ); bathophenanthroline (abbreviated to BPhen);bathocuproin (abbreviated to BCP);4,4-bis(5-methylbenzoxazol-2-yl)stilbene (abbreviated to BzOs); and thelike can be given, as well as metal complexes such astris(8-quinolinolato)aluminum (abbreviated to Alq₃);tris(4-methyl-8-quinolinolato)aluminum (abbreviated to Almq₃);bis(10-hydroxybenzo[h]-quinolinato)berylium (BeBq₂);bis(2-methyl-8-quinolinolato)-4-phenylphenolato-aluminum (abbreviated toBAlq); bis[2-(2-hydroxyphenyl)benzoxazolate]zinc (abbreviated toZn(BOX)₂); and bis[2-(2-hydroxyphenyl)benzothiazolato]zinc (abbreviatedto Zn(BTZ)₂). In addition, the electron-transporting layer 123 may alsobe a multilayer where two or more layers formed of the above substancesare combined.

An electron injecting layer may be provided between the second electrode102 and the electron transporting layer 123 as shown in FIG. 1. Here,the electron-injecting layer is a layer having a function to assistelectrons to be injected to the electron-transporting layer 123 from thesecond electrode 102. By providing the electron-injecting layer, theelectron affinity difference between the second electrode 102 and theelectron-transporting layer 123 is relieved; thus, electrons are easilyinjected. The electron-injecting layer is preferably formed using asubstance of which electron affinity is higher than that of a substanceforming the electron-transporting layer 123 and lower than that of asubstance forming the second electrode 102 or using a substance of whichenergy band curves by being provided as a thin film of 1 nm to 2 nmbetween the electron-transporting layer 123 and the second electrode102. As a specific example of a substance that can be used to form theelectron-injecting layer: inorganic matter such as alkaline metal,alkaline earth metal, fluoride of alkaline metal, fluoride of alkalineearth metal, oxide of alkaline metal, or oxide of alkaline earth metalcan be given. In addition to the inorganic matter, a substance that canbe used to form the electron-transporting layer 123 such as BPhen, BCP,BCP, p-EtTAZ, TAZ, or BzOs can also be used as a substance for formingthe electron-injecting layer by selecting a substance of which electronaffinity is larger than that of a substance for forming theelectron-transporting layer 123 from these substances. In other words,by selecting a substance such that the electron affinity in theelectron-injecting layer is comparatively higher than the electronaffinity in the electron transporting layer 123, the electron-injectinglayer can be formed. Note that in the case of providing theelectron-injecting layer, the second electrode 102 is preferably formedusing a substance having a lower work function such as aluminum.

Note that the hole-transporting layer 122 and the electron-transportinglayer 123 may be each formed by using a bipolar substance in addition tothe above substances. The bipolar substance indicates a substance inwhich mobility of either carrier of an electron or a hole is comparedwith mobility of the other carrier, a ratio value of one carriermobility to the other carrier mobility is 100 or less, preferably 10 orless. As for the bipolar substance, for example,2,3-bis(4-diphenylaminophenyl)quinoxaline (abbreviated to TPAQn);2,3-bis{4-[N-(1-naphthyl)-N-phenylamino]phenyl}-dibenzo[f,h]quinoxaline(abbreviated to NPADiBzQn); and the like can be given. It is preferableto particularly use a substance of which hole and electron mobility is1×10⁻⁶ cm²/Vs or more among the bipolar substances. In addition, thehole-transporting layer 122 and the electron-transporting layer 123 maybe formed by using the same bipolar substance.

Next, a case where the first layer 121 serves as a charge generatinglayer is described with reference to FIG. 14. In this case, alight-emitting element operates as follows: a voltage is applied to thefirst electrode 101 and the second electrode 102 such that a potentialof the first electrode 101 is higher than a potential of the secondelectrode 102. At this time, holes are injected into the first layer 121side from the first electrode 101 side, and electrons are injected intothe first layer 121 side from the second electrode 102 side. Inaddition, electrons moves from the first layer 121 to the firstelectrode 101, and holes moves from the first layer 121 to the secondelectrode 102. In each the light-emitting layer 111 a and the secondlight-emitting layer 111 b, the electrons and holes are recombined togenerate excitation energy. By the generated excitation energy, thelight-emitting substance is excited and when it returns to a groundstate, light is emitted.

When the first layer 121 serves as a charge generating layer, the firstlayer 121 is preferably a stacked layer of the first layer 121 a and thefirst layer 121 b. It is preferable that the first layer 121 a isprovided closer to that second electrode 102 than the first layer 121 b,and is a mixed layer of a metal oxide and an organic matter. As themetal oxide, metal oxides such as molybdenum oxide, vanadium oxide,ruthenium oxide, or rhenium oxide are preferable. However, other thanthe above, metal oxides such as titanium oxide, chromium oxide,zirconium oxide, hafnium oxide, tantalum oxide, tungsten oxide, andsilver oxide may also be used. As the organic matter, a substanceshowing an electron donating property to such metal oxides ispreferable, aromatic amine compound having a triphenylamine skeleton arepreferable in particular, such as4,4′-bis[N-(1-naphthyl)-N-phenylamino]biphenyl (abbreviated to NPB);4,4′-bis[N-(3-methylphenyl]-N-phenylamino]biphenyl (abbreviated to TPD);4,4′,4″-tris(N,N-diphenylamino)triphenylamine (abbreviated to TDATA);4,4′,4″-tris[N-(3-methylphenyl)-N-phenylamino]triphenylamine(abbreviated to MTDATA); and4,4′-bis{N-[4-(N,N-di-m-tolylamino)phenyl]-N-phenylamino}biphenyl(abbreviated to DNTPD); 1,3,5-tris[N,N-di(m-tolyl)amino]benzene(abbreviated to m-MTDAB); 4,4′,4″-tris(N-carbazolyl)-triphenylamine(abbreviated to TCTA). The first layer 121 a having the above structurecan generate holes.

In addition, when the first layer 121 serves as the charge generatinglayer, the first layer 121 b is preferably a layer formed by mixing atleast one substance selected from substances having higher mobility ofelectrons than that of holes and bipolar substances, with a substancethat shows an electron-donating property to the substances. Here, as forthe substance having higher mobility of electrons than that of holes,the same substance as the substance that can be used to form theelectron-transporting layer can be used. Moreover, as for the bipolarsubstance, the above bipolar substance such as TPAQn can be used.Further, as the substance that shows an electron-donating property, asubstance selected from alkaline metals or alkaline earth metals,specifically lithium (Li), calcium (Ca), sodium (Na), magnesium (Mg), orthe like can be used. In addition, alkaline metal oxide, alkaline earthmetal oxide, alkaline metal nitride or alkaline earth metal nitride,specifically at least one substance of lithium oxide (Li₂O), calciumoxide (CaO), sodium oxide (Na₂O), potassium oxide (K₂O), magnesium oxide(MgO), and the like can also be used. Moreover, alkaline metal fluorideor alkaline earth metal fluoride, specifically at least one substance oflithium fluoride (LiF), cesium fluoride (CsF), calcium fluoride (CaF₂),and the like can also be used as the substance that showselectron-donating properties. The first layer 121 b having the abovestructure can generate electrons.

An electron transporting layer is preferably formed between the firstlayer 121 b and the first light-emitting layer 111 a. Note that as forthe mode of the electron transporting layer, the description about theelectron transporting layer 123 described above is referred to. Further,a hole transporting layer is preferably formed between the first layer121 a and the second light-emitting layer 111 b. Note that as for themode of the hole transporting layer, the description about the holetransporting layer 122 described above is referred to.

Note that there are no particular limitations on the first electrode 101and the second electrode 102; however, at least one of the firstelectrode 101 and the second electrode 102 is preferably using aconductor which can be used as a transparent electrode such as indiumtin oxide, indium tin oxide containing silicon, or indium oxidecontaining zinc oxide at 2 to 20%, so that it can transmit visiblelight. However, it is not limited to such conductors, and gold (Au),platinum (Pt), nickel (Ni), tungsten (W), chromium (Cr), molybdenum(Mo), iron (Fe), cobalt (Co), copper (Cu), palladium (Pd) or the likecan be used for the first electrode 101 or the second electrode 102, aslong as it can be formed to be thick enough to transmit visible light.In addition, aluminum, further, an alloy of magnesium and silver, analloy of aluminum and lithium or the like may be used.

Next, a second light-emitting element shown in FIG. 1(B) is described.The second light-emitting element shown in FIG. 1(B) has a plurality oflight-emitting layers between a pair of electrodes, similarly to thelight-emitting element shown in FIG. 1(A). A first light-emitting layer211 a and a second light-emitting layer 211 b are included between thefirst electrode 201 and the second electrode 202. A first layer 221 isprovided between the first light-emitting layer 211 a and the secondlight-emitting layer 211 b. In addition, between the firstlight-emitting layer 211 a and the first electrode 201, a holetransporting layer 222 is provided. Between the first light-emittinglayer 211 b and the second electrode 202, an electron transporting layer223 is provided. As for the modes of the first light-emitting layer 211a and the second light-emitting layer 211 b, the description of thefirst light-emitting layer 111 a and the second light-emitting layer 111b mentioned above is referred to respectively. Note that light-emittingsubstances thereof are different from those included in the firstlight-emitting layer 111 a and the second light-emitting layer 111 betc. Specifically, in the first light-emitting layer 211 a and thesecond light-emitting layer 211 b, substances exhibiting emissionspectrum with peaks at 500 to 550 nm such as N,N′-dimethylquinacridon(abbreviated to DMQd); coumarin 6; coumarin 545T; andtris(8-quinolinolato)aluminum (abbreviated to Alq₃) are included as alight-emitting substance. In addition to the substances which emitfluorescence, substances which emit phosphorescence, such astris(2-phenylpyridinato)iridium(III),(acetylacetonato)bis(2-phenylpyridinato)iridium(III) may be used as alight-emitting substance.

Note that as for the first layer 221, the hole transporting layer 222,and the electron transporting layer 223, the description of the firstlayer 121, the hole transporting layer 122 and the electron transportinglayer 123 described above is refer to respectively.

Next, a third light-emitting element shown in FIG. 1(C) is described.The third light-emitting element shown in FIG. 1(C) has a plurality oflight-emitting layers between a pair of electrodes, in a similar mannerto the light-emitting element shown in FIG. 1(A). It has a firstlight-emitting layer 311 a and a second light-emitting layer 311 bbetween the first electrode 301 and the second electrode 302. A firstlayer 321 is provided between the first light-emitting layer 311 a andthe second light-emitting layer 311 b. Further, a hole transportinglayer 322 is provided between the first light-emitting layer 311 a andthe first electrode 301. In addition, an electron transporting layer 323is included between the second light-emitting layer 311 b and the secondelectrode 302. As for the modes of the first light-emitting layer 311 aand the second light-emitting layer 311 b, the description of the firstlight-emitting layer 111 a and the second light-emitting layer 111 bmentioned above may be referred to respectively. However, light-emittingsubstances thereof are different from those included in the firstlight-emitting layer 111 a and the second light-emitting layer 111 betc. Specifically, a substance exhibiting emission spectrum with peaksat 420 to 470 nm can be employed, such as9,10-bis(2-naphthyl)-tert-butylanthracene (abbreviated to t-BuDNA);9,9′-bianthryl; 9,10-diphenylanthracene (abbreviated to DPA); or9,10-bis(2-naphthyl)anthracene (abbreviated to DNA) is included in eachof the first light-emitting layer 311 a and the second light-emittinglayer 311 b. In addition to the substances which emit fluorescence, asubstance which emits phosphorescence, such asbis[2-(4,6-difluorophenyl)pyridinato](tetrapyrazolylboronato)iridium(III),may be used as a light-emitting substance.

Note that as for the first layer 321, the hole transporting layer 322,and the electron transporting layer 323, the description of the firstlayer 121, the hole transporting layer 122 and the electron transportinglayer 123 are referred to respectively.

In the light-emitting devices as described above, the firstlight-emitting layer 111 a (or the first light-emitting layer 211 a orthe first light-emitting layer 311 a) and the second light-emittinglayer 111 b (or the second light-emitting layer 211 b or the secondlight-emitting layer 311 b) emit light. That is, since light can beextracted at the same time from both the first light-emitting layer 111a (or the first light-emitting layer 211 a or the first light-emittinglayer 311 a) and the second light-emitting layer 111 b (or the secondlight-emitting layer 211 b or the second light-emitting layer 311 b),luminance of emission taken out is increased in accordance with thecurrent flowing to the light-emitting element. Consequently, a vividdisplay with high luminance can be performed.

Embodiment Mode 2

A light-emitting device of the present invention is not limited to themodes in which all light-emitting devices shown in FIGS. 1(A) to 1(C)have a plurality of light-emitting layers are provided between a pair ofelectrodes. For example, as shown in FIGS. 2(A) to 2(C), as for thesecond light-emitting element which exhibits emission color of lightwhich is highly sensible to human eyes, like a greenish emission, thenumber of light-emitting layers included between a pair of electrodesmay be smaller than that in a light-emitting element which emits areddish emission or a bluish emission. In this manner, when an image isdisplayed by combining emissions from light-emitting elements eachhaving a different luminance obtained when a predetermined currentflows, the number of light-emitting layers between a pair of electrodesis made different between a light-emitting element which emits emissioncolor of light which is less sensible to human eyes and a light-emittingelement which emits emission color of light which is highly sensible tohuman eyes, thereby harmonizing the luminances more efficiently.

As for FIGS. 2(A) to 2(C), FIGS. 2(A) and 2(C) are identical to FIGS.1(A) and 1(C), respectively and thus, the description thereof isomitted.

In this embodiment mode, a second light-emitting element shown in FIG.2(B) is described. The second light-emitting element shown in FIG. 2(B)has a light-emitting layer 261 between the first electrode 251 and thesecond electrode 252. Such a light-emitting element operates as follows.When a voltage is applied to the first electrode 251 and the secondelectrode 252 such that a potential of the first electrode 251 becomeshigher than a potential of the second electrode 252, holes are injectedto the light-emitting layer 261 from the first electrode 251 side, andelectrons are injected to the light-emitting layer 261 from the secondelectrode 252 side. Electrons and holes are recombined in thelight-emitting layer 261 to generate excitation energy. A light-emittingsubstance included in the first light-emitting layer 261 is excited bythe generated excitation energy and then, light is emitted when itreturns to a ground state.

In the light-emitting layer 261, a light-emitting substance which canexhibit a greenish emission only or such a light-emitting substancedispersed in a substance having a larger energy gap than thelight-emitting substance is included. As specific examples of alight-emitting substance, light-emitting substances exhibiting emissionspectrum with peaks at 500 to 550 nm such as N,N′-dimethylquinacridon(abbreviated to DMQd); coumarin 6; coumarin 545T; andtris(8-quinolinolate)aluminum (abbreviated to Alq₃) can be given. Inaddition to the substances which emit fluorescence, substances whichemit phosphorescence, such as tris(2-phenylpyridinato) iridium (III),(acetylacetonato)bis(2-phenylpyridinato)iridium(III) can be used as alight-emitting substance.

As shown in FIG. 2(B), a hole transporting layer 272 is preferablyprovided between the light-emitting layer 261 and the first electrode251. In addition, an electron transporting layer 273 is preferablyprovided between the light-emitting layer 261 and the second electrode252. Note that as for the mode of the electron transporting layer 273,the description of the electron transporting layer 123 described beforeis referred to. In addition, as for the mode of the hole transportinglayer 272, the description of the hole transporting layer 122 describedabove is referred to.

Embodiment Mode 3

In a light-emitting device of the present invention, light emitted fromlight-emitting substances included in the first light-emitting layer andthe second light-emitting layer may have the same color as described inEmbodiment Mode 1, or may have different colors respectively. Like this,a mode of a light-emitting element in which light emission from thelight-emitting substances included in the first light-emitting layer andthe second light-emitting layer have different emission colors isdescribed with reference to FIG. 3.

In FIG. 3, a first light-emitting layer 411 a and a secondlight-emitting layer 411 b are provided between the first electrode 401and the second electrode 402. There is no particular limitation onemission color of a light-emitting substance included in each of thefirst light-emitting layer 411 a and the second light-emitting layer 411b; however, this embodiment mode describes a mode in which alight-emitting substance which emits reddish light is included in thefirst light-emitting layer 411 a, and a light-emitting substance whichemits blue-greenish light is included in the second light-emitting layer411 b.

In the first light-emitting layer 411 a, a substance exhibiting emissionspectrum with peaks at 600 to 680 nm can be employed:4-dicyanomethylene-2-isopropyl-6-[2-(1,1,7,7-tetramethyljulolidin-9-yl)ethenyl]-4H-pyran(abbreviated to DCJTI);4-dicyanomethylene-2-methyl-6-[2-(1,1,7,7-tetramethyljulolidin-9-yl)ethenyl]-4H-pyran(abbreviated to DCJT);4-dicyanomethylene-2-tert-butyl-6-[2-(1,1,7,7-tetramethyljulolidin-9-yl)ethenyl]-4H-pyran(abbreviated to DCJTB); periflanthene;2,5-dicyano-1,4-bis[2-(10-methoxy-1,1,7,7-tetramethyljulolidin-9-yl)ethenyl]benzeneor the like is included as the light-emitting substance. In addition tothe substances which emit fluorescence, substances which emitphosphorescence, such as,(acetylacetonato)bis[2-(2-benzothienyl)pyridinato]iridium(III)(abbreviated to Ir(btp)₂(acac)),(2,3,7,8,12,13,17,18-octaetyl-21H,23H-porphyrinato)platinum(II) may beincluded as a light-emitting substance.

Note that in the first light-emitting layer 411 a, a light-emittingsubstance only or such a light-emitting substance dispersed in asubstance having a larger energy gap than the light-emitting substanceis included.

In the second light-emitting layer 411 b, a substance which emits ablue-greenish light is included as a light-emitting substance, such asbis(2-methyl-8-quinolinolato)(4-phenylphenolato)aluminum (abbreviated toBAlq), bis(2-methyl-8-quinolinolato)(4-phenylphenolato)gallium(abbreviated to BGaq), or3,6,11,14-tetrakis(diphenylamino)dibenzo[g,p]chrysene. In addition tosuch substances which emit fluorescence, a substance which emitsphosphorescence, such asbis{2-[3,5-bis(trifluoromethyl)phenyl]pyridinato}(picolonato)iridium(III),can be used as a light-emitting substance.

Note that in the second light-emitting layer 411 b, a light-emittingsubstance only or such a light-emitting substance dispersed in asubstance having a larger energy gap than the light-emitting substanceis included.

A first layer 421 is provided between the first light-emitting layer 411a and the second light-emitting layer 411 b. As for the first layer 421,the description of the first layer 121 described above is referred to.

Between the first light-emitting layer 411 a and the first electrode401, a hole transporting layer 422 may be provided as shown in FIG. 3.In addition, an electron transporting layer 423 may be provided betweenthe second light-emitting layer 411 b and the second electrode 402, asshown in FIG. 3. Note that the description of the hole transportinglayer 122 and the electron transporting layer 123 is referred to as forthe hole transporting layer 422 and the electron transporting layer 423respectively.

Note also that the description of the first electrode 101 and the secondelectrode 102 is referred to as for the first electrode 401 and thesecond electrode 402 respectively.

It is be noted that either the first electrode or the second electrode402 may be formed to have a stacked structure of a layer made of ahighly reflective conductor such as aluminum, and a layer made of aconductor which can transmit visible light, such as indium tin oxide.Further, in the case of employing such a structure, by changing thethickness of the layer which can transmit visible light, an optical pathin which emitted light goes may be adjusted to obtain emission with ahighly color purity by utilizing the interference effect of light.

In the light-emitting device described in this embodiment mode, by thesame operation of the light-emitting device described in Embodiment Mode1, light emission from the first light-emitting layer 411 a and thesecond light-emitting layer 411 b can be obtained. Light emission formthe first light-emitting layer 411 a and the second light-emitting layer411 b are visible by mixing emission colors of the light emission whenlight is extracted from either the first electrode 401 or the secondelectrode 402, or from the both electrodes. As in this embodiment mode,in a case that reddish light is emitted from the first light-emittinglayer 411 a, and blue-greenish light is emitted from the second emittingelement 411 b, the light is visible as white light by mixing the colors.

Embodiment Mode 4

Although Embodiment Mode 3 is described on the mode of thelight-emitting element including two light-emitting layers eachexhibiting a different emission color between a pair of electrodes, thenumber of light-emitting layers included between a pair of electrodesmay be three, for example.

One mode of a light-emitting device of the present invention isdescribed with reference to FIG. 4.

FIG. 4 shows a light-emitting element including a first light-emittinglayer 511, a second light-emitting layer 512, and a third light-emittinglayer 513 between a first electrode 501 and a second electrode 502. Alight-emitting substance which exhibits reddish light emission isincluded in the first light-emitting layer 511, a light-emittingsubstance which exhibits greenish light emission is included in thesecond light-emitting layer 512, and a light-emitting substance whichexhibits bluish light emission is included in the third light-emittinglayer 513. As for specific examples of a light-emitting substance whichexhibits each light emission colors, the description made in EmbodimentMode 1 is referred to.

Between the first light-emitting layer 511 and the second light-emittinglayer 512, a first layer 521 is provided, and between the secondlight-emitting layer 512 and the third light-emitting layer 513, asecond layer 524 is provided. In the light-emitting element shown inthis embodiment mode, the first layer 521 and the second layer 524 eachserve as a charge generating layer. The first layer 521 is formed bystacking a first layer 521 a and a first layer 521 b. The second layer524 is formed by stacking a second layer 524 a and a second layer 524 b.The first layer 521 a is provided closer to the second electrode 502than the first layer 521 b. In addition, the second layer 524 a isprovided closer to the second electrode 502 than the second layer 524 b.Each of the first layer 521 a and the second layer 524 a is a layergenerating holes, which is includes a metal oxide and an organic mattersimilarly to the first layer 121 a described before. As specificexplanation of the first layer 521 a and the second layer 524 a, thedescription of the first layer 121 a is referred to. In addition, eachof the first layer 521 b and the second layer 524 b is a layergenerating electrons, similarly to the first layer 121 b describedbefore. As specific explanation of the first layer 521 b and the secondlayer 524 b, the description of the first layer 121 b is referred to.

A hole transporting layer 522 may be provided between the firstlight-emitting layer 511 and the first electrode 501 as shown in FIG. 4.In addition, an electron transporting layer 523 may be provided betweenthe third light-emitting layer 513 and the second electrode 502 as shownin FIG. 4. Note that the description of the hole transporting layer 122and the electron transporting layer 123 is referred to as for the holetransporting layer 522 and the electron transporting layer 523respectively.

In the light-emitting element of this embodiment mode, a hole injectinglayer may be provided between the hole transporting layer 522 and thefirst electrode 501. In addition, an electron injecting layer may beprovided between the second electrode 502 and the electron transportinglayer 523. Note that as for the electron injecting layer and the holeinjecting layer, the description about them in Embodiment Mode 1 isreferred to respectively.

In addition to those described above, a hole transporting layer, anelectron transporting layer or the like may be provided appropriately.

In the light-emitting element shown in FIG. 4, when a voltage is appliedsuch that a potential of the second electrode 502 becomes lower than apotential of the first electrode 501, holes are injected to the firstlight-emitting layer 511 from the first electrode 501 side, andelectrons are injected to the third light-emitting layer 513 from thesecond electrode 502 side. The electrons are transported from the firstlayer 521 b to the first light-emitting layer 511, and the holes aretransported from the first layer 521 a to the light-emitting layer 512.In addition, the electrons are transported from the second layer 524 bto the second light-emitting layer 512, and the holes are transportedfrom the second layer 524 b to the third light-emitting layer 513 side.In each of the first light-emitting layer 511, the second light-emittinglayer 512 and the third light-emitting layer 513, electrons and holesare recombined to generate excitation energy. A light-emitting substanceis excited by the generated excitation energy, and then, light isemitted when it returns to a ground state.

Reddish light emitted from the first light-emitting layer 511, greenishlight emitted from the second light-emitting layer 512 and bluish lightemitted from the third light-emitting layer 513 are taken out to theoutside through either or both of the first electrode 501 and the secondelectrode 502. These lights are mixed visually and recognized as whitelight.

Note that the description of the first electrode 101 and the secondelectrode 102 is referred to as for the first electrode 501 and thesecond electrode 502 respectively.

A light-emitting element described above is included in thelight-emitting device of the present invention.

Embodiment Mode 5

One mode of a light-emitting device of the present invention isdescribed with reference to FIGS. 5(A) and 5(B). FIG. 5(B) is a viewshowing a broken line portion of FIG. 5(A). In the light-emitting deviceof the present invention, a second light-emitting element 602, a thirdlight-emitting element 603 and a fourth light-emitting element 604 areprovided in addition to the first light-emitting element 601. Theselight-emitting elements are formed by providing a light-emitting layerbetween a pair of electrodes. One of the three light-emitting elements,the second light-emitting element, the third light-emitting element andthe fourth light-emitting element, exhibits reddish emission, another ofthem exhibits greenish emission and the other of them exhibits bluishemission. Here, as one example, described is a case where the secondlight-emitting element is for reddish emission, the third light-emittingelement is for greenish emission, and the fourth light-emitting elementis for bluish emission; however, the order is not limited. In addition,each of the second light-emitting element 602, the third light-emittingelement 603 and the fourth light-emitting element 604 is arranged to beadjacent with the first light-emitting element. Specifically, they arearranged as shown in FIG. 5(A).

In FIG. 5(A), each of the first light-emitting element 601, the secondlight-emitting element 602, the third light-emitting element 603 and thefourth light-emitting element 604 is formed to have a square shape.However, the shape of each light-emitting element is not limited to thatshown in FIG. 5(A), and for example, a circular shape may be adopted.The second light-emitting element 602 is provided on the first side ofthe first light-emitting element 601. The third light-emitting element603 is provided on the second side of the first light-emitting element601. The fourth light-emitting element 604 is provided on the third sideof the first light-emitting element 601.

Stacked structures of these light-emitting elements are described withreference to FIGS. 6(A) and 6(B). FIG. 6(A) is a cross-sectional view ofa portion taken along the broken line A-B in FIG. 5(B), and FIG. 6(B) isa cross-sectional view of a portion taken along the broken line C-D inFIG. 5. In FIG. 6(A), the first electrode 612, the first light-emittinglayer 622, an electron generation layer 631, and the second electrode615 are stacked sequentially, and by combining them in this manner, theyserve as the second light-emitting element 602. In addition, the firstelectrode 614, the third light-emitting layer 624, and the electrongeneration layer 634 are stacked sequentially, and by combining them inthis manner, they serve as the fourth light-emitting element 604. InFIG. 6(B), the first electrode 613, the hole generation layer 632, thesecond light-emitting layer 623, the electron generation layer 633, thehole generation layer 634, and the second electrode 615 are sequentiallyformed, and by combining them in this manner, they serve as the thirdlight-emitting element 603. The first electrode 611, the firstlight-emitting layer 622, the electron generation layer 631, the holegeneration layer 632, the second light-emitting layer 623, the electrongeneration layer 633, the hole generation layer 634, the thirdlight-emitting layer 624, the electron generation layer 635, and thesecond electrode 615 are stacked sequentially. By combining them in thismanner, they serve as the first light-emitting element 601.

The first light-emitting element 601, the second light-emitting element602, the third light-emitting element 603 and the fourth light-emittingelement 604 which are formed as described above, serve as one set ofpixels.

Further, in this embodiment mode, the first light-emitting layer 601 isformed to include a layer which is provided to be common with each ofthe second light-emitting element 602, the third light-emitting element603 and the fourth light-emitting element 604. Top views of FIGS. 7(A)to 7(D) show how the layers exhibiting each emission color are formed.FIGS. 7(A) to 7(C) are views showing how each of the firstlight-emitting layer 622, the second light-emitting layer 623 and thethird light-emitting layer 624 is arranged. FIG. 7(D) is a view in whichFIGS. 7(A) to 7(C) are overlapped. From the FIGS. 7(A) to 7(D), it canbe understood that a stacked region is provided in each of the firstlight-emitting layer 622, the second light-emitting layer 623 and thethird light-emitting layer 624. As described above, a layer exhibitingreddish emission included in the first light-emitting layer 601 isformed at the same time when the second light-emitting element 602 isformed. A layer exhibiting greenish emission included in the firstlight-emitting layer 601 is formed at the same time when the thirdlight-emitting element 603 is formed. A layer exhibiting bluish emissionincluded in the first light-emitting layer 601 is formed at the sametime when the fourth light-emitting element 604 is formed. Consequently,a light-emitting device can be manufactured with fewer steps than thecase of manufacturing the first light-emitting element 601 in anotherstep independent from the manufacturing step of the other light-emittingelements. White emission can be obtained from the first light-emittingelement.

Note that in FIGS. 6(A) and 6(B), a transistor 651 which is provided todrive a light-emitting element and which is electrically connected to alight-emitting element, is also shown in addition to the light-emittingelement. The transistor shown in FIG. 6(A) shows a schematiccross-sectional structure seen from a direction perpendicular to thedirection of current flow. The transistor 651 is provided over asubstrate 652 similarly to the light-emitting element. In addition, thelight-emitting element and the transistor 651 are provided in differentlayers to sandwich an insulating layer 653 therebetween. End portions ofthe first electrodes 611, 612, 613 and 614 included in the firstlight-emitting element 601 to the fourth light-emitting element 604 areeach covered with a partition layer 655. Note that the partition layer655 is formed from an organic insulator or an inorganic insulator. Byproviding the partition layer 655, a short-circuit between electrodes ofa light-emitting element caused by the influence from an electric fieldconcentrated on the end portions of the first electrode, can beprevented.

A hole generation layer may be provided between the first electrode 611,612 and the first light-emitting layer 622, and/or between the firstelectrode 614 and the third light-emitting layer 624. In addition, ahole generation layer may be provided between the electron generationlayer 631 and/or 635 and the second electrode 615.

Embodiment Mode 6

In a light-emitting device of the present invention, a circuit forcontrolling driving of the light-emitting element may be provided, aswell as each light-emitting element described in Embodiment Modes 1 to5.

In FIGS. 8(A) and 8(B), a transistor 702, a wire and the like areprovided over a substrate 701. The transistor 702 is electricallyconnected to the light-emitting element 703. By a signal inputted to thecircuit over the substrate 701, emission/non-emission, emission time, acurrent value to be supplied to a light-emitting element, or the like isdetermined for each light-emitting element. And emission andnon-emission are combined in plural light-emitting elements provided ina light-emitting device to display an image.

The light-emitting device is preferably sealed so as not to expose thelight-emitting element to the atmosphere. There is no particularlimitation on a sealing method. For example, as shown in FIG. 8, thesubstrate 701 and a substrate 711 are attached and sealed by a sealingmaterial 712 such that the light-emitting element 703 is encapsulatedinside. In addition, the sealed inside may be filled with a nitrogen gasor a gas such as an inert gas, or may be filled with a resin or thelike. FIG. 9 shows one mode of a sealed light-emitting device in which aspacer 813 is provided over a light-emitting element and a partitionlayer 804 which is provided between light-emitting elements and issealed so as not to damage the light-emitting element 803 by the contactof a substrate 811 and the light-emitting element 803. In addition, inthe light-emitting device shown in FIG. 9, a filter 814 is provided tobe overlapped with the light-emitting element 803. Light emitted fromthe light-emitting element 803 passes through the filter 814 to becolored with a desired color, and is extracted to the outside of thelight-emitting device. There is no particular limitation on whether thefilter 814 is provided or not. Further, in the light-emitting deviceshown in FIG. 9, the substrate 801 and the substrate 811 are attached bya resin 821, and the light-emitting element 803 is encapsulated so asnot to be exposed to the atmosphere. Note that light may be extractedfrom the opposite substrates of the light-emitting device having thestructure shown in FIG. 9, as shown in FIG. 13(A).

In the light-emitting device as shown in FIG. 8 or FIG. 9, a signal sentfrom the outside is transmitted to a circuit provided in thelight-emitting device through a wire 705 or a wire 805 which is lead outfrom the inside of the light-emitting device to the outside of thelight-emitting device. The wire 705 and the wire 805 are connected to aflexible print circuit 716 and a flexible print circuit 816 by aconductive adhesive 715 and a conductive adhesive 815, respectively.

Moreover, the light-emitting device may be manufactured by attaching twosubstrates 901 and 902 provided with a light-emitting element and acircuit to encapsulate the light-emitting element 903 and thelight-emitting element 911 provided over each substrate inside, as shownin FIG. 13B. In the light-emitting device as shown in FIG. 13, differentor the same images can be taken out from the both substrates.

Embodiment Mode 7

FIG. 10 shows a module in which a light-emitting device 7001 and acircuit board 7002 are combined. In the circuit board 7002, for example,a control circuit 7014, a signal division circuit 7015 or the like isprovided. The light-emitting device 7001 includes at least one oflight-emitting elements described in Embodiment Mode 1 to EmbodimentMode 5, and is sealed as in Embodiment Mode 6.

This light-emitting device 7001 is provided with a pixel portion 7005 inwhich a light-emitting element is provided in each pixel, a scanningline driver circuit 7006, and a signal line driver circuit 7007 whichsupplies a video signal to a selected pixel. In addition, a signal istransmitted to the light-emitting device 7001 from the circuit board7002 through a connection wiring 7008.

By mounting this module, a television image receiver can be completed.FIG. 11 is a block diagram showing a main constitution of the televisionimage receiver. A tuner 7011 receives video signals and audio signals.The video signal is processed by a video signal amplifier circuit 7012,a video signal processing circuit 7013 that converts a signal outputtedfrom that into a color signal corresponding to each color, and a controlcircuit 7014 that converts the video signal to specifications of adriver IC. The control circuit 7014 outputs a signal to each of the scanline side and the signal line side. In the case of digital driving, asignal division circuit 7015 may be provided on the signal line side, sothat an inputted digital signal is divided into m signals to besupplied.

Among signals received by the tuner 7011, an audio signal is transmittedto an audio signal amplifier circuit (audio wave amplifier circuit)7016, and an output thereof is supplied to a speaker 7018 through anaudio signal processing circuit 7017. A control circuit 7019 receivescontrol information of a receiving station (received frequency) and asound volume from an input portion 7020, and transmits signals to thetuner 7011 and the sound signal processing circuit 7017.

As shown in FIG. 12, a television image receiver can be completed byincorporating a module in a casing 7031. A display screen 7032 is formedusing a module. within addition, a speaker 7033, an operating switch7034 and the like is provided appropriately.

Since this television image receiver includes the light-emitting device7001, a vivid image with excellent quality can be displayed.

As described above, a television image receiver which can provide afavorable image, can be provided by incorporating a light-emittingdevice of the present invention, such that it can serve as a displayportion. In addition, it may be incorporated in a telephone as well as atelevision image receiver. Therefore, a mobile phone, a televisiontelephone or the like which can provide a favorable image, can becompleted.

Embodiment 1

An example of a pixel applicable to the light-emitting devices describedin Embodiment Modes 1 to 6 is shown in FIG. 15. Although only one pixelis shown here, a plurality of pixels are arranged in matrix in a rowdirection and a column direction in a pixel portion of thelight-emitting device in reality.

A pixel has a switching transistor 1001, a driving transistor 1002, acapacitor element 1003, a light-emitting element 1004, a scanning line1005, a signal line 1006, and a power source line 1007. A gate terminalof the switching transistor 1001 is connected to the scanning line 1005,a first terminal (a source terminal or a drain terminal) thereof isconnected to the signal line 1006, and a second terminal (the sourceterminal or the drain terminal) thereof is connected to a gate terminalof the driving transistor 1002. Further, the second terminal of theswitching transistor 1001 is connected to the power source line 1007through the capacitor element 1003. Further, a first terminal (a sourceterminal or a drain terminal) of the driving transistor 1002 isconnected to the power source line 1007 and a second terminal (thesource terminal or the drain terminal) thereof is connected to a firstelectrode of the light-emitting element 1004. To a second electrode ofthe light-emitting element 1004, a low power source potential is set. Itis to be noted that a low power source potential is, based on a highpower source potential which is set to the power source line 1007, apotential satisfying the relation of the low power source potential<thehigh power source potential, and for example, GND, 0 V, or the like maybe set as the low power source potential. Note that the capacitorelement 1003 may be omitted by substituting gate capacitance of thedriving transistor 1002. Note that a pixel configuration is not limitedto this.

Here, a driving method of a pixel provided with the light-emittingelement of the present invention is described using a voltage inputtingvoltage driving method and a voltage inputting current driving method asexamples.

First, described is a voltage inputting voltage driving method with thepixel configuration shown in FIG. 15.

When the pixel is selected by the scanning line 1005, that is, when theswitching transistor 1001 is on, a video signal is inputted from thesignal line 1006 to the pixel. Then, a charge for a voltagecorresponding to the video signal is accumulated in the capacitorelement 1003, and the capacitor element 1003 holds the voltage. Thisvoltage is a voltage between the gate terminal and the first terminal ofthe driving transistor 1002, which corresponds to a gate-source voltageVgs of the driving transistor 1002.

Generally, an operating region of a transistor can be divided into alinear region and a saturation region. The border is when (Vgs−Vth)=Vdsis satisfied where a drain-source voltage is denoted by Vds, agate-source voltage is denoted by Vgs, and a threshold voltage isdenoted by Vth. In the case of satisfying (Vgs−Vth)>Vds, the transistoroperates in a linear region and a current value thereof is dependent onlevels of Vds and Vgs. Meanwhile, in the case of satisfying(Vgs−Vth)<Vds, the transistor operates in a saturation region andideally, a current value thereof hardly changes even when Vds changes.That is, the current value is determined only by the level of Vgs.

Here, in the case of the voltage inputting voltage driving method, avideo signal is inputted such that Vgs of this driving transistor 1002becomes two states of sufficiently turning on and turning off thedriving transistor 1002. That is, the driving transistor 1002 isoperated in a linear region.

Therefore, in the case of a video signal of turning on the drivingtransistor 1002, ideally, a power source potential Vdd which has beenset to the power source line 1007 can be set to the first electrode ofthe light-emitting element 1004 as it is.

That is, ideally, a voltage applied to the light-emitting element 1004is made constant so that the luminance obtained from the light-emittingelement 1004 becomes constant. Then, a plurality of subframe periods areprovided in one frame period, a writing of a video signal to a pixel isperformed per subframe period to control light emission/non-lightemission of the pixel per subframe period, so that gradation isexpressed depending on the total of a subframe period in which lightemission is performed.

On the other hand, in a voltage inputting current driving method, thedriving transistor 1002 is operated in a saturation region.

Then, depending on a value of Vgs of the driving transistor 1002, acurrent value flowing to the driving transistor 1002 is determined. Thatis, based on a video signal inputted to the capacitor element 1003 fromthe signal line 1006, the luminance of the light-emitting element 1004is determined to express gradation.

Here, in the case where a light-emitting device performs a full-colordisplay which is composed of color elements of R (red), G (green), and B(blue), an applied voltage or an applied current to a light-emittingelement in order to obtain a required luminance for the light-emittingelement is different for respective pixels of the color elements of R,G, and B. Described next is, thus, a method of changing a voltageapplied to a light-emitting element per pixel of each color element. Inparticular, as described in Embodiment Modes 1 to 5, an applied voltagerequired for a pixel having a light-emitting element including twolight-emitting layers is high since it is necessary to apply a voltageto each of the two light-emitting layers.

First, description is made on the case of the voltage inputting voltagedriving method. In order to change an applied voltage to thelight-emitting element 1004 for respective pixels of the color elements,a potential which is set to the power source line 1007 for each colorelement may be changed. In addition, in the case of changing thepotential which is set to the power source line 1007 for each colorelement, the driving transistor 1002 may be turned on/off sufficiently.That is, in the case of increasing a potential of the power source line1007 connected to the first terminal which is a source terminal of thedriving transistor 1002, a gate potential required for turning on thedriving transistor 1002 sufficiently (a video signal of L level) may behigher by the potential increase of the power source line 1007 while agate potential required for turning off the driving transistor 1002sufficiently (a signal of H level) is required to be higher by thepotential increase of the power source line 1007. That is, when thepotential of the power source line 1007 is increased, respectivepotentials of H level and L level of a video signal required for turningon/off the driving transistor 1002 become higher by that. Note that thesignal of L level is not necessarily increased because operation can beperformed even if the signal of L level remains low. On the other hand,as for a video signal of a pixel of a color element in the case ofdecreasing the potential of the power source line 1007, respectivepotentials of H level and L level of the video signal required forturning on/off the driving transistor 1002 become lower by that. Notethat the signal of H level is not necessarily decreased becauseoperation can be performed even if the signal of H level remains high.

Next, shown in FIG. 16 is a signal line driver circuit for shifting thelevel of a video signal per color element of the pixels. A video signaloutputted from this signal line driver circuit is written into thepixel, and in accordance with the video signal, on/off of the drivingtransistor 1002 which controls on/off of the light-emitting element 1004is controlled.

By a sampling pulse outputted from a pulse output circuit 1101, a switch(one of a switch 1111, a switch 1112, a switch 1113, and the like)provided in a signal line (one of Sr, Sg, Sb, and the like) for eachcolumn of the pixels is turned on sequentially. Then, through the switchwhich is turned on, the video signal is inputted to a level shifter (oneof a level shifter 1121, a level shifter 1122, a level shifter 1123, andthe like). That is, when the switch 1111 is turned on, the video signalis inputted to the level shifter 1121, when the switch 1112 is turnedon, the video signal is inputted to the level shifter 1122, and when theswitch 1113 is turned on, the video signal is inputted to the levelshifter 1123.

It is to be noted that a latch circuit may be provided between the levelshifter and the video signal line. One latch circuit may be provided ora plurality (for example, two) of latch circuits may be provided persignal line (Sr, Sg, Sb, or the like).

Then, in the case where the video signal is inputted to the levelshifter 1121, the level of the signal is set such that H level of thevideo signal is VHr and L level thereof is VLr. This level may be setbased on a potential of the power source line 1007 of a pixel of a colorelement R. For example, in the case where the light-emitting element1004 of the pixel of the color element R includes two or morelight-emitting layers and a required applied voltage is large, thepotential of the power source line 1007 is made high and the level ofthe video signal is also shifted to be high. On the other hand, in thecase where the required applied voltage the light-emitting element 1004of the pixel of the color element R is low, the potential of the powersource line 1007 is made low and the level of the video signal is alsoshifted to be low. Note here that being high or low of the requiredapplied voltage of the light-emitting element 1004 means whether it ishigher or lower as compared to the light-emitting element 1004 of apixel of another color element.

Meanwhile, in the case where the video signal is inputted to the levelshifter 1122, the level of the signal is set such that H level of thevideo signal is VHg and L level thereof is VLg. In addition, in the casewhere the video signal is inputted to the level shifter 1123, the levelof the signal is set such that H level of the video signal is VHb and Llevel thereof is VLb.

The video signal of which the level is shifted by the level shifter 1121is inputted to a buffer 1131, and the current supply capability isincreased by the buffer 1131 to be outputted to the signal line Sr. Thebuffer outputs a signal having about the same potential as the signalinputted thereto. The signal line Sr corresponds to the signal line 1006of the pixel in FIG. 15. Similarly, the video signal of which the levelis shifted by the level shifter 1122 is inputted to a buffer 1132, andthe current supply capability is increased by the buffer 1132 to beoutputted to the signal line Sg. The video signal of which the level isshifted by the level shifter 1123 is inputted to a buffer 1133, and thecurrent supply capability is increased by the buffer 1133 to beoutputted to the signal line Sb.

In this manner, the video signals of which the levels are set per colorelement of R, G, or B are written into pixels of a selected row. Notethat through the signal line Sr, the signal is inputted to the pixel ofa color element of R. Through the signal lines Sg and Sb, the signalsare inputted to the pixels of color elements of G and B respectively.

In this manner, the video signal is inputted to the pixels of theselected row sequentially through the signal line of a column of whichthe switch is turned on.

In the case where the size (a lighting period or the like) of a videosignal is corrected in order to change an applied voltage of thelight-emitting element for respective pixels of the color elements,gamma correction may also be performed; a gamma value is stored in amemory means in advance to form a table. Then, the video signal iscorrected in accordance with the gamma value.

Noted that, in the voltage inputting voltage driving method, when oneframe period is divided into a plurality of subframes to expressgradation (a digital time gray scale method), the frame frequencybecomes high since a large number of gray scale levels are expressed.Therefore, in the case of expressing the large number of gray scalelevels, display with the large number of gray scale levels can beperformed while suppressing increase of the frame frequency, bycombining an area gray scale method and the digital time gray scalemethod. For example, for expressing 1024 gray scale levels, fivesub-pixels are arranged in one pixel in which they have the area ratioof 2⁰, 2¹, 2², 2³, and 2⁴, and one frame is divided into five subframeshaving the ratio of 2⁰, 2¹, 2², 2³, and 2⁴. Then, by combining them,1024 gray scale levels can be expressed; that is, the sub-pixel havingthe smallest area may be emitted light during the shortest subframeperiod in the case of gradation of the least significant bit. On theother hand, in the case of gradation of the most significant bit, allthe sub-pixels within one pixel may be emitted light in all thesubframes.

Next, description is made on the case of the voltage inputting currentdriving method with reference to FIG. 17. A schematic diagram of alight-emitting device with the voltage inputting current driving methodis shown in in FIG. 17. The light-emitting device includes a pulseoutput circuit 1201 and a pixel portion 1202. In the pixel portion 1202,a plurality of pixels 1203 are arranged in matrix. Note that the pixelshown in FIG. 15 can apply to the pixel 1203.

By a sampling pulse outputted from the pulse output circuit, a switch(one of 1205 r, 1205 g, 1205 b, and the like) provided in a signal line(one of Sr, Sg, Sb, and the like) for each column of the pixels isturned on sequentially. Then, through the switch which is turned on, thevideo signal is inputted to the signal line for each column of thepixels 1203. That is, when the switch 1205 r is turned on, the videosignal is inputted to the signal line Sr, when the switch 1205 g isturned on, the video signal is inputted to the signal line Sg, and whenthe switch 1205 b is turned on, the video signal is inputted to thesignal line Sb. Then, the signal is written into pixels of a selectedrow. Note that through the signal line Sr, the signal is inputted to thepixel of a color element of R. Through the signal lines Sg and Sb, thesignal is inputted to the pixels of color elements of G and Brespectively.

In this manner, the video signal is inputted to the pixels of theselected row sequentially through the signal line of a column of whichthe switch is turned on.

In the case of the voltage inputting current driving method, a voltagevalue corresponding to this video signal is different depending on thegray scale level of the pixel. Note that a potential is set to thesignal line when inputting a voltage.

Therefore, a potential of the video signal is corrected depending onpixels for each color element. That is, even in a case of a video signalat the same gray scale level, the potential may be changed depending oneach color element of R, G, and B. When a required applied current ofthe light-emitting element is large, the potential of the video signalis made lower than a video signal at the same gray scale level ofanother color element. Thus, a current flowing into the drivingtransistor is increased. That is, a current flowing into thelight-emitting element is increased.

Here, described is a method of shifting a potential of the video signalat each gray scale level depending on each color element. Note thatdescribed in this embodiment mode is a method of shifting the videosignal at each gray scale level depending on each color element, using adigital-analog conversion circuit.

FIG. 23 shows a resistor string digital-analog conversion circuit with 8gray scale levels (a 3-bit).

A resistor element group 1601 in which resistors are connected inseries, a first switch group 1602, a second switch group 1603, and athird switch group 1604 are included.

A high reference power source potential V_(Href) is set to one terminalof the resistor element group 1601 and a low reference power sourcepotential V_(Lref) is set to the other terminal thereof. Into an inputterminal 3 which is an input terminal of the third switch group 1604, asignal of the third bit is inputted to select one of a group of grayscale levels 0 to 3 and a group of gray scale levels of 4 to 7. Into aninput terminal 2 which is an input terminal of the second switch group1603, a signal of the second bit is inputted to select one of a group ofgray scale levels 0, 1, 4, and 5 or a group of gray scale levels of 2,3, 6, and 7. Into an input terminal 1 which is an input terminal of thefirst switch group 1602, a signal of the first bit is inputted to selectone of a group of gray scale levels 0, 2, 4, and 6 or a group of grayscale levels of 1, 3, 5, and 7. In this manner, a video signalcorresponding to the gray scale level belonging to all the groupsselected in the first switch group 1602, the second switch group 1603,and the third switch group 1604, is outputted from an output terminalV_(OUT).

Note that a potential outputted from the output terminal V_(OUT) isoutputted as a potential by resistance-dividing a potential differencebetween the high reference power source potential V_(Href) and the lowreference power source potential V_(Lref) which are set to the terminalsof the resistor element group in which connection is performed inseries, depending on the number of gray scale levels.

Therefore, in this embodiment, a value of the reference power sourcepotential which is set to the digital-analog conversion circuit ischanged for each color element of RGB. That is, when a video signalwhich is to be inputted to a pixel of the color element of which anapplied voltage required for a light-emitting element is high isconverted into an analog signal, a value of the low reference powersource potential V_(Lref) of the digital-analog conversion circuit ismade lower than the low reference power source potential of the othercolor elements. On the contrary, when a video signal which is to beinputted to a pixel of the color element of which an applied voltagerequired for a light-emitting element is low is converted into an analogsignal, a value of the low reference power source potential V_(Lref) ofthe digital-analog conversion circuit is made higher than the lowreference power source potential of the other color elements.

In addition, in the case where an N-channel type transistor is adoptedas the driving transistor, when a video signal which is to be inputtedto a pixel of the color element of which an applied voltage required fora light-emitting element is high is converted into an analog signal, avalue of the high reference power source potential V_(Href) of thedigital-analog conversion circuit is made higher than the high referencepower source potential of the other color elements. On the contrary,when a video signal which is to be inputted to a pixel of the colorelement of which an applied voltage required for a light-emittingelement is low is converted into an analog signal, a value of the highreference power source potential V_(Href) of the digital-analogconversion circuit is made lower than the high reference power sourcepotential of the other color elements.

In this manner, the video signal can be corrected for each colorelement.

In the case where the voltage value of a video signal is corrected inorder to change an applied voltage of the light-emitting element forrespective pixels of the color elements, gamma correction may also beperformed; a gamma value is stored in a memory means in advance to forma table. Then, the video signal is corrected in accordance with thegamma value.

It is to be noted that a “switch” described in this embodiment or thelike is not limited particularly, such as an electrical switch or amechanical switch, as long as it can control as current flow. Forexample, a transistor, a diode, a logic circuit combining them, or thelike may be used. Therefore, in the case of using a transistor as aswitch, polarity (conductivity) thereof is not particularly limitedbecause it operates just as a switch. However, when an off current ispreferred to be small, a transistor with a polarity with a small offcurrent is favorably used. For example, the transistor which has an LDDregion has a small off current. Further, it is desirable that ann-channel type transistor be employed when a potential of a sourceterminal of the transistor as a switch is closer to the potential of alow potential side power source (Vss, Vgnd, 0 V or the like), and ap-channel type transistor be employed when the potential of the sourceterminal is closer to the potential of a high potential side powersource (Vdd or the like). This is because it can be easily operated as aswitch since an absolute value of the gate-source voltage can beincreased. Note also that a CMOS type switch may be used as well byusing both n-channel and p-channel type transistors.

As described above, the video signal is an analog value. In addition, inthe case of a pixel of which a light-emitting element includes two ormore light-emitting layers, a required voltage value of a video signalis high. Therefore, a signal which becomes a sampling pulse from thepulse output circuit, for controlling on/off of the switch 1205 r, theswitch 1205 g, and the switch 1205 b in order to set an analog value tothe signal lines (Sr, Sg, and Sb), may be changed by a level shifter1206 r, a level shifter 1206 g, and a level shifter 1206 b. That is, inthe case where a transistor is used as the switch 1205 r, the switch1205 g, and the switch 1205 b, the transistors functioning as the switch1205 r, the switch 1205 g, and the switch 1205 b for selecting pixels tobe written a video signal are easily operated in a linear region even ifa value of a video signal is largely different for respective pixels ofthe color elements.

That is, in the case where a P-channel type transistor is used as thedriving transistor of the pixel 1203, if an applied potential requiredfor the light-emitting element is higher as compared to pixels of theother color elements, a potential of a video signal is required to belower than a potential of a video signal at the same gray scale level ofthe pixels of the other color elements. Consequently, a potential of asource terminal of the transistor functioning as the switch (the 1205 r,the 1205 g, the 1205 b, or the like) for controlling supply of the videosignal to the signal line becomes largely different per color element ofa pixel connected to the signal line. Note that in order to input ananalog video signal into a pixel, a drain terminal of the transistorfunctioning as the switch for controlling supply of the video signal tothe signal line is connected to a pixel side.

Here, in order to operate the transistor as a switch, it is requiredthat Vgs<Vth is satisfied when the switch is off whereas both Vgs>Vthand Vds<Vgs−Vth are satisfied when the switch is on. Note that Vgs is agate-source voltage of a transistor, Vds is a drain-source voltage ofthe transistor, and Vth is a threshold voltage of the transistor.

Accordingly, in the case where a potential of the source terminal of thetransistor as the switch for controlling supply of the video signal tothe signal line is largely different per color element of pixels, alevel of the sampling pulse which is inputted to the gate terminalthereof is also preferably changed per color element of a pixelconnected to the signal line.

Moreover, not only the potential of the video signal but also apotential of the power source line may be changed per color element. Anexample of a pixel configuration denoted by a dotted line 1204 is shownin FIG. 18. The common reference numerals are used for the commonportions of FIG. 15. In the pixel configuration in FIG. 18, signal linesSr, Sg, and Sb are correspond to those in FIG. 17 respectively. Inaddition, as a power source line corresponding to the power source line1007 in FIG. 15, power source lines Vr, Vg, and Vb are provided for apixel 1301R of a color element R, a pixel 1301G of a color element G,and a pixel 1301B of a color element B respectively. In addition, thescanning line 1005 shown in FIG. 15 corresponds to a scanning line Gi inFIG. 18.

It is to be noted that in the case where the potential of the powersource line is changed such as the power source lines Vr, Vg, and Vb,the level of the potential of the video signal for making a pixel emitno-light is also changed. For example, when the potential of the powersource line Vr of a pixel of a color element R is increased, thepotential of a source terminal of the driving transistor 1002 iscorrespondingly increased, therefore, the potential of a video signal ofH level required for turning off the driving transistor 1002 is alsoincreased. That can be similarly applied to respective elements of thecolor elements of G and B.

Note that a second electrode of the light-emitting element 1004 shown inFIG. 18 is a common electrode to respective pixels of the color elementsof RGB. Therefore, in the case where a common potential is set to thepower source lines Vr, Vg, and Vb which are provided for respectivepixels of the color elements of RGB, and a voltage to be applied to thelight-emitting element 1004 for respective pixels of the color elementsof RGB is controlled only by a video signal, there occurs the followingproblem if the difference of the applied voltage required for thelight-emitting element 1004 is large.

That is, in a pixel where the applied voltage required for thelight-emitting element 1004 is small, the drain-source voltage of thedriving transistor 1002 becomes large so that the speed of deterioratingthe driving transistor 1002 becomes fast. On the other hand, in a pixelwhere the applied voltage required for the light-emitting element 1004is large, the drain-source voltage of the driving transistor 1002becomes too small so that the operation is performed in the linearregion.

Details thereof are described below with reference to FIG. 24. FIG. 24is a schematic diagram of respective elements of color elements of RGB.A pixel 1701 is a pixel of the color element of R, a pixel 1702 is apixel of the color element of G, and a pixel 1703 is a pixel of thecolor element of B.

Each of the pixel 1701, the pixel 1702, and the pixel 1703 includes adriving transistor and a light-emitting element. A drain-source voltageof the driving transistor of the pixel 1701 is denoted by V_(ds1), anapplied voltage of the light-emitting element thereof is V_(ELR), and anapplied current of the same is I_(R), a drain-source voltage of thedriving transistor of the pixel 1702 is denoted by V_(ds2), an appliedvoltage of the light-emitting element thereof is V_(ELG), and an appliedcurrent of the same is I_(G), and a drain-source voltage of the drivingtransistor of the pixel 1703 is denoted by V_(ds3), an applied voltageof the light-emitting element thereof is V_(ELB), and an applied currentof the same is I_(B).

It is assumed here that a large voltage is required as the appliedvoltage V_(ELR) whereas a small voltage is required as the appliedvoltage V_(ELB). That is, the required applied voltages satisfy therelation of V_(ELR)>>V_(ELG)>>V_(ELB). Thus, in the case of setting theapplied voltage to the light-emitting element as described above only bya video signal, respective drain-source voltages of the drivingtransistors becomes V_(ds1)<<V_(ds2)<<V_(ds3) since a high power sourcepotential Vdd and a low power source potential are common amongrespective pixels of the color elements.

Consequently, the drain-source voltage V_(ds3) of the driving transistorof the pixel of the color element of B becomes too large. Accordingly,the transistor is destroyed.

Meanwhile, the drain-source voltage V_(ds1) of the driving transistor ofthe pixel of the color element of R becomes too small. Accordingly, thedriving transistor is operated in the linear region.

Therefore, it is found that a power source potential is preferably setfor each color element of potentials of RGB, in order to solve theproblem.

Accordingly, in this embodiment, a video signal is inputted to thesignal lines Sr, Sg, and Sb by being corrected arbitrarily forrespective pixels of the color elements, even in the case of the pixelsat the same gray scale level. In addition, by setting a high powersource potential to the power source lines Vr, Vg, and Vb for respectivepixels of the color elements, an applied voltage of the light-emittingelement 1004 for obtaining the luminance suitable for respective pixelsof the color elements can be easily set.

Note that the second electrode of the light-emitting element 1004 may bechanged for each color as well.

In addition, described in this embodiment is the method in which thelevel of the video signal for each gray scale level is shifted bychanging the high reference power source potential or the low referencepower source potential with the resistor string digital-analogconversion circuit (R-DAC); however, both the high reference powersource potential and the low reference power source potential may bechanged to shift the level of the video signal for each gray scale levelas well. In addition, in stead of the resistor string digital-analogconversion circuit, a capacitor array digital conversion circuit (C-DAC)may be used, or the capacitor array digital conversion circuit and theresistor string digital conversion circuit may be used in combination aswell. In that case also, the potential of the video signal for each grayscale level can be shifted depending on respective pixels of the colorelements by changing the reference power source potential arbitrarily.

Note that the description is made on the case of a dot sequential methodas for the writing from the signal line to the pixel in this embodiment;however, a line sequential method may be of course employed as well inwhich video signals for one row are latched and the latched videosignals are written all at once to pixels for one row.

In addition, the description is made on the case of adopting a P-channeltype transistor as the driving transistor in this embodiment; however,an N-channel type transistor may be of course adopted as well. In thatcase, respective levels of the video signal for making the pixel emitlight and emit no-light are inverted. That is, the pixel emits light inthe case of H level whereas emits no-light in the case of L level.

Embodiment 2

A circuit for generating different high power source potentials isdescribed with reference to FIG. 19 in this embodiment. In thisembodiment, a voltage step-up circuit, so-called a charge pump is used.

A constant voltage source 1400 for supplying a voltage of Vref; acapacitor element 1401, a capacitor element 1402, a capacitor element1403, a capacitor element 1404, a rectifying element 1411, a rectifyingelement 1412, a rectifying element 1413, and a rectifying element 1414are included. The rectifying element 1411, the rectifying element 1412,the rectifying element 1413, and the rectifying element 1414 areconnected in series, and an input terminal of the rectifying element1411 is connected to a high potential side of the constant voltagesource 1400. Then, a low potential side of the constant voltage source1400 is connected to GND. An output terminal of the rectifying element1411 is connected to an input terminal of the rectifying element 1412 atan intersection point 1421, an output terminal of the rectifying element1412 is connected to an input terminal of the rectifying element 1413 atan intersection point 1422, and an output terminal of the rectifyingelement 1413 is connected to an input terminal of the rectifying element1414 at an intersection point 1423. In addition, a first electrode ofthe capacitor element 1401 is connected to the intersection point 1421,and a clock signal (CLK) is inputted to a second electrode thereof. Inaddition, a first electrode of the capacitor element 1402 is connectedto the intersection point 1422, and a clock inverted signal (CLKB) isinputted to a second electrode thereof. In addition, a first electrodeof the capacitor element 1403 is connected to the intersection point1423, and a clock signal (CLK) is inputted to a second electrodethereof. In addition, a first electrode of the capacitor element 1404 isconnected to an output terminal 1424 of the rectifying element 1414, asecond electrode thereof is connected to GND. Note that anotherpredetermined potential may be inputted to the second electrode of thecapacitor element 1404 as well.

Note that the intersection point 1421 is a first output terminal, theintersection point 1422 is a second output terminal, the intersectionpoint 1423 is a third output terminal, and the output terminal 1424 ofthe rectifying element 1414 is a fourth output terminal.

Here, in the case where the L level of the clock signal (CLK) and theclock inverted signal (CLKB) is GND and the H level thereof is v, byinputting the clock signal and the clock inverted signal continuously, acharge is accumulated in the capacitor elements 1401, 1402, 1403, and1404 so that the potential of the intersection point 1421 becomes thepotential of Vref and a potential of Vref+v alternately.

Consequently, the potential of Vref or the potential of Vref+v, thepotential of Vref+2 v or the potential of Vref+v, and the potential ofVref+2 v or the potential of Vref+3 v can be obtained from the firstoutput terminal, the second output terminal, and the third outputterminal respectively. Note that the potential of Vref+3 v can beobtained from the fourth output terminal.

Therefore, an arbitrarily potential may be obtained by the potentialobtained from each of the first to fourth output terminals, through astabilized power source circuit (regulator). A potential of Vref+v orless, a potential of Vref+2 v or less, and a potential of Vref+3 v orless can be obtained from the first output terminal, the second outputterminal, and the third output terminal and the fourth output terminalrespectively. Therefore, for example, arbitrarily potentials in therange of Vref to Vref+v, in the range of Vref+v to Vref+2 v, in therange of Vref+2 v to Vref+3 v, and in the range of Vref+2 v to Vref+3 vare preferably obtained from the first output terminal, the secondoutput terminal, the third output terminal, and the fourth outputterminal respectively.

It is to be noted that the clock pulse (CLK) or the clock inverted pulse(CLKB) which is inputted to the second electrodes of the capacitorelements 1401, 1402, and 1403, or the like is not necessarily inputtedall of the time, and for example, it may be stopped being inputted whenthe potential of the fourth output terminal becomes a certain potential.

A structure in the case where inputting of the clock pulse (CLK) or theclock inverted pulse (CLKB) is stopped when a condition is satisfied isdescribed with reference to a simple schematic diagram shown in FIG. 25.

The potential of Vref is supplied to an input terminal of a charge pump1801 from a constant voltage source 1800, so that different potentialscan be obtained from the first to fourth output terminals respectively.Here, a potential detecting circuit 1803 detects the potential of thefourth output terminal which obtains the output from the rectifyingelement at the farthest point, and outputs a control signal when thepotential becomes a certain potential so that the clock pulse (CLK) orthe clock inverted pulse (CLKB) is stopped being inputted from a clockpulse generating circuit 1802.

Next, a structure for using each potential obtained from the firstoutput terminal to the fourth output terminal as a stabilized powersource is described.

Shown in FIG. 20 is a structure to which a stabilized power source forsupplying four power source potentials can be applied. The commonreference numerals are used for the common structures of FIG. 19. In thestructure shown in FIG. 20, potentials obtained from the intersectionpoint 1421, the intersection point 1422, and the intersection point 1423are made to be constant potentials by a stabilized power source circuit1431, a stabilized power source circuit 1432, and a stabilized powersource circuit 1433 respectively. In addition, In order to make thepotential of the output terminal 1424 of the rectifying element 1414constant, the capacitor element 1404 with large electrostaticcapacitance is used. As a result, since a charge capable of being storedbecomes large, the fluctuation of the potential can be suppressed.

Accordingly, respective potentials obtained from the output terminals 1to 4 can be stabilized.

In addition, before obtaining the output from the output terminal, oneelectrode of a capacitor with large capacitance for accumulating theoutput is connected through a rectifying element, and the other elementthereof is connected to GND. Then, an output may be obtained from theone electrode side of the capacitor element. Accordingly, thefluctuation of the output potential can be suppressed.

That is, as the stabilized power source circuits 1431, 1432, and 1433,they can be constituted by a rectifying element and a capacitor elementas shown in FIG. 21. The stabilized power source circuit 1431 isconstituted by a rectifying element 1441 and a capacitor element 1451,the stabilized power source circuit 1432 is constituted by a rectifyingelement 1442 and a capacitor element 1452, and the stabilized powersource circuit 1433 is constituted by a rectifying element 1443 and acapacitor element 1453.

Note that described here is an operation of the stabilized power sourcecircuit constituted by the rectifying element 1441 and the capacitorelement 1451.

As described above, the potential of the intersection point 1421 becomesthe potential of Vref and the potential of Vref+v alternately. Then,when the potential of the intersection point 1421 is Vref+v, a currentflows into the rectifying element 1441 and a charge is accumulated inthe capacitor element 1451. On the other hand, when the potential of theintersection point 1421 is Vref, a current does not flow since a voltageapplied to the rectifying element 1441 is a reverse-biased voltage.Therefore, a current is supplied from the intersection point 1421 suchthat a charge for a certain voltage is accumulated in the capacitorelement 1451. In addition, even if the charge accumulated in thecapacitor element 1451 is discharged from the first output terminal, byincreasing the electrostatic capacitance of the capacitor element 1451,the fluctuation of the potential obtained from the first output terminalcan be suppressed.

Note that as for the second output terminal and the third outputterminal, they are similar to the above, and thus, the descriptionthereof is omitted herein.

Other constitutions of a stabilized power source circuit (regulator)applicable to the stabilized power source circuit 1431, the stabilizedpower source circuit 1432, and the stabilized power source circuit 1433are shown in FIGS. 22(a) and 22(b). According to the configurations ofFIGS. 22(a) and 22(b), respective predetermined potentials which arelower than the highest potentials obtained from the intersection points1421, 1422, and 1423 can be obtained from the first to third outputterminals.

The configuration of FIG. 22(a) is described. Before obtaining theoutput from the output terminal, one terminal of a zener diode isconnected, and the other element thereof is connected to GND.Consequently, a current flows to the zener diode when the potential ofthe output exceeds a certain potential, so that the potential of theoutput terminal can be adjusted.

A stabilized power source circuit 1504 is constituted by a rectifyingelement 1501, a capacitor element 1502, and a zener diode 1503.

An unstabilized potential is inputted from an input terminal of thestabilized power source circuit 1504. Then, when the unstabilizedpotential is H level, a current flows to the rectifying element 1501 anda charge is accumulated in the capacitor element 1502. That is, therectifying element 1501 supplies a current to the capacitor element 1502so as to be the potential of H level of the unstabilized potential. Thatis, a charge is accumulated in the capacitor element 1502. In addition,by connecting the zener diode 1503 to the capacitor element 1502 inparallel, the accumulated charge is discharged when a voltage betweenelectrodes of the capacitor element 1502 exceeds a certain voltage. Thatis, the zener diode 1503 functions to reduce the voltage when thevoltage between the electrodes of the capacitor element 1502 exceeds thecertain voltage.

In addition, the capacitor element 1502 with large electrostaticcapacitance is used. In this manner, the voltage between the electrodesof the capacitor element 1502 can be kept to be constant and from anoutput terminal of the stabilized power source circuit 1504, a constantpotential can be obtained.

Note that the input terminal of the stabilized power source circuit 1504can be connected to any one of the intersection points 1421, 1422, and1423, or the like in FIG. 20, and the output terminal of the stabilizedpower source circuit 1504 can be any one of the first output terminal,the second output terminal, and the third output terminal of the chargepump in FIG. 20.

In addition, the stabilized power source circuit 1504 can be applied tothe stabilized power source circuits 1431, 1432, and 1433 in FIG. 20;however, it can also be used for stabilizing the output from the fourthoutput terminal. In that case, the rectifying element 1501 and thecapacitor element 1502 correspond to the rectifying element 1414 and thecapacitor element 1404 in FIG. 20 respectively. Therefore, by providingthe zener diode 1503 so as to be in parallel to the capacitor element1404, the potential of the fourth output terminal can be an arbitrarypotential.

In addition, the number of the zener diodes 1503 is not limited to oneand a plurality thereof may be provided in series to adjust thepotential. For example, in the stabilized power source circuits 1431,1432, and 1433, the output terminal 1424, and the like, a plurality ofzener diodes may be provided in accordance with respective potentials.In addition, zener diodes of which breakdown potentials are differentmay be connected in series to adjust the potential as well.

Next, a stabilized power source circuit 1517 shown in FIG. 22(b) isdescribed.

The stabilized power source circuit 1517 is constituted by a rectifyingelement 1511, a capacitor element 1512, an amplifier 1513, a firstresistor 1515, and a second resistor 1516. An input terminal of thestabilized power source circuit 1517 is connected to the intersectionpoint 1421, 1422, or 1423, or the like. The potential of theintersection point 1421, 1422, or 1423, or the like becomes H level andL level in accordance with the timing of the clock pulse (CLK) or theclock inverted pulse0 (CLKB). Therefore, as a potential which isinputted to the input terminal of the stabilized power source circuit1517, the potential of H level or L level is inputted. Then, when thepotential of the input terminal is H level, a current flows to therectifying element 1511 and a charge is accumulated in the capacitorelement 1512. In this manner, a current is supplied from the rectifyingelement 1511 such that a voltage between electrodes of the capacitorelement 1512 becomes constant.

This voltage between the electrodes of the capacitor element 1512 isused as a power source of the amplifier 1513. To a non-inverted inputterminal of the amplifier 1513, a constant voltage is inputted from areference power source 1514. In addition, an inverted input terminal ofthe amplifier 1513 is connected to an output terminal through the secondresistor 1516, and further, the inverted input terminal is connected toa ground power source GND through the resistor 1515. Note that ahigh-gain amplifier is used as the amplifier 1513.

In the amplifier 1513, a voltage of the reference power source 1514 isinputted to the non-inverted input terminal whereas a voltage byresistance-dividing an output voltage of the amplifier 1513 with thesecond resistor 1516 and the first resistor 1515 is inputted to theinverted input terminal, and these voltage values are compared to eachother.

An output voltage V₀ of the amplifier 1513 can be expressed by thefollowing formula (1) where the voltage of the reference power source1514 is V_(r), a resistance value of the first resistor 1515 is R₁, anda resistance value of the second resistor 1516 is R₂.

$\begin{matrix}{V_{0} = {V_{r}\frac{{R\; 1} + {R\; 2}}{R\; 1}}} & (1)\end{matrix}$

Accordingly, an output potential of the amplifier 1513 can be adjustedby a rate R₂/R₁ between the capacitance values of the first resistor1515 and the second resistor 1516. That is, to the output terminal ofthe stabilized power source circuit 1517, a potential which is equal toor higher than the potential of the reference power source 1514 andequal to or lower than the potential of H level inputted to the inputterminal of the stabilized power source circuit 1517 can be arbitrarilyset.

Note that a plurality of FIG. 22(b) may be provided in the end of theoutput terminal 1424 to output a plurality of potentials as well.However, in the case of a low output potential, a large voltage isapplied to the amplifier 1513 so that a power consumption becomes large.Therefore, FIG. 22(b) may be provided in each terminal of the stabilizedpower source circuits 1431, 1432, and 1433 so that the most appropriatepotential may be outputted as well. As a result of this, the powerconsumption can be reduced.

Further, the potential may be obtained from each of the first to fourthoutput terminals through a smoothing circuit instead of the regulator aswell.

In addition, a means for detecting the output potential may be providedbefore obtaining an output from the output terminal. Then, when theoutput potential exceeds a certain set potential, inputting of the clocksignal (CLK) or the clock inverted signal (CLKB) inputted to the secondelectrode of the capacitor element may be stopped.

It is to be noted that the description is made based on the assumptionthat the rectifying elements 1411, 1412, 1413, and 1414 are idealrectifying elements of which a forward threshold voltage is 0 V. Inreality, when there is a forward threshold voltage Vth of the rectifyingelement 1411, the potential of the intersection point 1421 becomes apotential of Vref−Vth or a potential of Vref−Vth+v. In addition, sincedecrease occurs by the threshold voltage per the rectifying elementinterposed therebetween, the threshold voltage of the rectifying elementis desirably low. Note that a circuit for correcting for the thresholdvoltage of the rectifying element may be provided.

As the rectifying element, a diode-connected transistor, a PN junctionor PIN junction diode, a Schottky diode, or a carbon nanotube diode canbe used.

Alternatively, a switch which is turned on only when a forward bias ofthe rectifying element is applied, in synchronism with the clock signal(CLK) or the clock inverted signal (CLKB) may be used as the rectifyingelement. As the switch, a transistor can be used.

Note that it is needless to say that the numbers of the rectifyingelements and the capacitor elements, and the amplitudes of the clocksignal and the clock inverted signal may be arbitrarily set to adjustthe potential obtained from the output terminal arbitrarily. Inaddition, the output terminal may be between arbitrary rectifyingelements in the rectifying elements connected in series. Therefore, theoutput terminal provided is not necessarily provided depending onbetween the rectifying elements.

It is to be noted that the output from the voltage step-up circuit ofthis constitution can be used for the high power source potentialdescribed in Embodiment 1 (for example, VHr, VHg, YHb, or the like).

In addition, the description is made on the charge pump for generating adifferent plurality of high power source potentials; however, adifferent plurality of low power source potentials may be generated aswell. In that case, the direction of the rectifying elements connectedin series are reversed as shown in FIG. 27 so that the high potentialside and the low potential side of the constant voltage source arereversed.

A capacitor element 2001, a capacitor element 2002, a capacitor element2003, a capacitor element 2004, a rectifying element 2011, a rectifyingelement 2012, a rectifying element 2013, and a rectifying element 2014are included. The rectifying element 2011, the rectifying element 2012,the rectifying element 2013, and the rectifying element 2014 areconnected in series, and an output terminal of the rectifying element2011 is connected to GND. An input terminal of the rectifying element2011 is connected to an output terminal of the rectifying element 2012at an intersection point 2021, an input terminal of the rectifyingelement 2012 is connected to an output terminal of the rectifyingelement 2013 at an intersection point 2022, and an input terminal of therectifying element 2013 is connected to an output terminal of therectifying element 2014 at the intersection point 1423. In addition, afirst electrode of the capacitor element 2001 is connected to theintersection point 2021, and to a second electrode thereof, the clocksignal (CLK) is inputted. In addition, a first electrode of thecapacitor element 2002 is connected to the intersection point 2022, andto a second electrode thereof, the clock inverted signal (CLKB) isinputted. In addition, a first electrode of the capacitor element 2003is connected to the intersection point 2023, and to a second electrodethereof, the clock signal (CLK) is inputted. In addition, a firstelectrode of the capacitor element 2004 is connected to an inputterminal 2024 of the rectifying element 2014 and a second terminalthereof is connected to GND.

Respective potentials obtained from the intersection point 2021, theintersection point 2022, and the intersection point 2023 are made to beconstant potentials by stabilized power source circuits 2031, 2032, and2033 to be the first output terminal, the second output terminal, andthe third output terminal. In addition, in order to make the potentialof the input terminal 2024 of the rectifying element 2014 constant, thecapacitor element 2004 with large electrostatic capacitance is used. Asa result, since a charge capable of being stored in the capacitorelement 2004 becomes large, the fluctuation of the potential can besuppressed. In addition, as the stabilized power source circuit(regulator), the constitution described as for the voltage step-upcircuit can be arbitrarily used.

Therefore, the output from this voltage step-down circuit can be usedfor the low power source potential described in Embodiment 1 (forexample, VLr, VLg, YLb, or the like).

In addition, the clock pulse (CLK) or the clock inverted pulse (CLKB) isinputted to the charge pump in this embodiment; however, a secondelectrode of a capacitor element 1902 is connected to a high potentialside of a constant voltage source 1901 through a first switch 1903 and alow potential side thereof is connected to GND. In addition, the secondelectrode of the capacitor element 1902 is connected to GND through aswitch 1904. Then, the switch 1903 and the switch 1904 may be turnedon/off alternatively. In this case, depending on whether the capacitorelement 1902 is the capacitor element 1401 or the capacitor element 1402shown in FIG. 20, the on/off of the switches 1903 and 1904 may bereversed.

That is, by using the charge pump described in this embodiment, anarbitrarily power source potential can be generated by a simpleconstitution. Accordingly, in the light-emitting devices described inEmbodiment Modes 1 to 6, respective voltage can be applied forlight-emitting elements of which emission colors are different.

What is claimed is:
 1. A light-emitting device comprising: a firstlight-emitting element comprising a first light-emitting layer between afirst electrode and a second electrode; a second light-emitting elementcomprising a second light-emitting layer between a third electrode and afourth electrode; a third light-emitting element comprising a thirdlight-emitting layer between a fifth electrode and a sixth electrode;and a fourth light-emitting element comprising the first light-emittinglayer, the second light-emitting layer, and the third light-emittinglayer between a seventh electrode and an eighth electrode, wherein thefirst light-emitting layer is shared by the first light-emitting elementand the fourth light-emitting element, wherein the first light-emittinglayer is not included in the second light-emitting element and the thirdlight-emitting element, wherein the first light-emitting layer isconfigured to emit a first color, wherein the second light-emittinglayer is configured to emit a second color, and wherein the thirdlight-emitting layer is configured to emit a third color.
 2. Thelight-emitting device according to claim 1, further comprising a chargegeneration layer between the first light-emitting layer and the secondlight-emitting layer in the fourth light-emitting element.
 3. Thelight-emitting device according to claim 1, wherein the firstlight-emitting element, the second light-emitting element, and thefourth light-emitting element are arranged in a line in a firstdirection, wherein the third light-emitting element and the fourthlight-emitting element are arranged in a line in a second direction, andwherein the first direction is vertical to the second direction.
 4. Thelight-emitting device according to claim 1, wherein the secondelectrode, the fourth electrode, the sixth electrode, and the eighthelectrode are the same layer.
 5. The light-emitting device according toclaim 1, wherein the second electrode, the fourth electrode, the sixthelectrode, and the eighth electrode are electrically connected to oneanother.
 6. The light-emitting device according to claim 1, wherein thefirst electrode, the third electrode, the fifth electrode, and theseventh electrode are separated from one another.
 7. The light-emittingdevice according to claim 1, wherein at least one of the firstlight-emitting layer, the second light-emitting layer, and the thirdlight-emitting layer comprises a substance exhibiting emission from atriplet excited state.
 8. The light-emitting device according to claim1, wherein the first color, the second color, and the third color arered, green and blue, respectively.
 9. The light-emitting deviceaccording to claim 1, wherein the fourth light-emitting element isconfigured to emit white light.
 10. A light-emitting device comprising:a first light-emitting element comprising a first light-emitting layerbetween a first electrode and a second electrode; a secondlight-emitting element comprising a second light-emitting layer betweena third electrode and a fourth electrode; and a third light-emittingelement comprising the first light-emitting layer and the secondlight-emitting layer between a fifth electrode and a sixth electrode,wherein the first light-emitting layer is shared by the firstlight-emitting element and the third light-emitting element, wherein thefirst light-emitting layer is not included in the second light-emittingelement, wherein the first light-emitting layer is configured to emit afirst color, and wherein the second light-emitting layer is configuredto emit a second color.
 11. The light-emitting device according to claim10, further comprising a charge generation layer between the firstlight-emitting layer and the second light-emitting layer in the thirdlight-emitting element.
 12. The light-emitting device according to claim10, wherein the first light-emitting element and the thirdlight-emitting element are arranged in a line in a first direction,wherein the second light-emitting element and the third light-emittingelement are arranged in a line in a second direction, and wherein thefirst direction is vertical to the second direction.
 13. Thelight-emitting device according to claim 10, wherein the secondelectrode, the fourth electrode, and the sixth electrode are the samelayer.
 14. The light-emitting device according to claim 10, wherein thesecond electrode, the fourth electrode, and the sixth electrode areelectrically connected to one another.
 15. The light-emitting deviceaccording to claim 10, wherein the first electrode, the third electrode,and the fifth electrode are separated from one another.
 16. Thelight-emitting device according to claim 10, wherein at least one of thefirst light-emitting layer and the second light-emitting layer comprisesa substance exhibiting emission from a triplet excited state.
 17. Thelight-emitting device according to claim 10, wherein the first color andthe second color are blue and yellow, respectively.
 18. Thelight-emitting device according to claim 10, wherein the thirdlight-emitting element is configured to emit white light.